could be explained as a result of mechanical pressure exerted by the elongated carapace, if only species of Architarbus and closely allied genera were known. But the same abbreviation is present in Geratarbus and Opiliotarbus, genera with a straight posterior edge of the carapace. Textfigure 2. Architarbus minor Petrunkevitch, specimen No. 189, Peabody Museum, Yale University. Type. View of ventral surface with abdominal tergites superimposed on one side to show their position relative to the corresponding sternites. The anterior edge of the first tergite is at the same time the posterior edge of the carapace. Genus Hadrachne Melander, 1903. New definition. Carapace slightly longer than wide, subtriangular, with convex sides and acutely pointed anterior end. Posterior edge of carapace widely and gently procurved. Eyes wanting. Other characters not certain on account of poor preservation. Genotype H.horribilis Melander. To this genus belongs probably Geraphrynus angustus Pocock, described from a single specimen from Coseley, near Dudley, England. Genus Leptotarbus, new. Definition of the genus. Carapace much longer than wide, the ratio being 5:3. Its posterior edge is evenly rounded, procurved; its sides gently converging and almost straight in posterior two thirds, then rapidly converging in a curve to a point in front. Eyes wanting. Sternum composed of three sclerites. Abdomen elongated, nearly twice as long as wide. Anterior five tergites abbreviated, but only the first and second very much so and the first tergite is clearly overlapped by the carapace. Only first and second tergites procurved. Genotype L.torpedo (Pocock), sub Geraphrynus. Genus Discotarbus Petrunkevitch, 1913. New definition. Carapace triangular, slightly wider than long, with convex sides, angular posterior corners and gently procurved posterior edge. Eyes wanting. Abdomen almost discshaped. Six anterior tergites abbreviated, first to third procurved, the others straight. Sternum long, composed of three sclerites. Pedipalpal coxae narrow, contiguous, reaching sternum. Entire body very flat. Genotype D.deplanatus Petrunkevitch. Discotarbus deplanatus Petrunkevitch. Trans. Conn. Acad., 1913, p. 121-122. Textfigures Specimen No. 14869, a and b, in the Illinois State Museum collection. This is a rather poorly preserved specimen which I refer to the above species. The piece labeled a presents the ventral surface and is convex, the piece labeled b presents the dorsal surface and is concave. Both are rather poorly preserved, the ventral surface somewhat better than the dorsal. Some pieces of appendages are visible, but neither their identity nor structure can be ascertained. The ventral surface shows a piece of the sternum, the third and fourth pedal coxae, the genital sternite with the genital opening and following this five or possibly six sternites and a faint impression of the anal operculum. The dorsal surface shows the carapace, badly pressed out of shape, but showing clearly the procurved posterior margin. Total length of the specimen 9.2 mms. Carapace 4.0 mms. long, 5.6 mms. wide. The angle between the hind coxae is ca.155°. Genus Metatarbus Petrunkevitch, 1913. New definition. Carapace triangular, wider than long, with convex sides and procurved posterior edge, forming rounded angles. Eyes wanting, but an elongated median elevation present in the place of an eyetubercle. Anterior four tergites abbreviated, but five tergites are divided by a longitudinal median line. First three tergites procurved, fourth tergite in the shape of a biconcave lens, the following one recurved. Abdomen with ten tergites and nine sternites. Two lines divide the fourth to seventh sternites into three fields. The eighth and ninth sternites are entire. Genotype M. triangularis Petrunkevitch. Metatarbus triangularis Petrunkevitch. Trans Conn. Acad., 1913, pp. 122-123, Textfigures Holotype No. 182 of the Peabody Museum of Yale University is the only specimen which I have reexamined, the Harvard Museum specimen mentioned in my Monograph being not as well preserved. The reexamination of the holotype after careful cleaning revealed structures which escaped observation before. The Textfigures 3 and 4 shown here superceed those of my Monograph as the new definition of the genus superceeds the old one. The carapace shows an elongated, shallow depression representing what was in life a low eleva Textfigure 3. Metatarbus triangularis Petrunkevitch, specimen No. 182, Type. Peabody Museum. Dorsal view. Anal operculum purposely omitted. This drawing shows features not figured in 1913. Textfigure 4. Metatarbus triangularis Petrunkevitch, specimen No. 182, Type. Peabody Museum. Ventral view. This drawing shows features discovered on reexamination and not figured in 1913. tion, but no eyes are present. As the drawing shows, five tergites are longitudinally divided by a median line, but the fifth is not abbreviated. The ventral surface shown in Textfigure 4 presents nine sternites (not seven as stated in the Monograph and shown in the old figure). Two round depressions are visible on the second sternite. They represent probably a pair of seminal receptacles, the genital opening remaining invisible. The discrepancy in the number of tergites and sternites is probably referable to the fusion of two sternites in the posterior region of the abdomen. The anal operculum is visible on both the obverse and the reverse of the nodule, but is omitted from the drawing represented by Textfigure 3, because it belongs to the ventral surface of the animal. The angle between the hind coxae is ca. 90°. Genus Paratarbus, new. Definition of the Genus. Carapace wider than long, triangular, with convex sides and an evenly curved posterior edge. Anteriorly the carapace is acutely pointed. Two sessile eyes present. Pedipalpal coxae small, wedged in between the first pedal coxae. Sternum very long and narrow. Abdomen with ten tergites and nine sternites. Six anterior tergites abbreviated and cut by a median line. Fourth to seventh sternites divided into three fields by two longitudinal lines. Genotype P.carbonarius, n.sp. Paratarbus carbonarius, new sp. Holotype No. 14864, a and b, of the collection of the Illinois State The holotype is the only specimen belonging to this species. It was originally heavily coated with a white calcarious mineral which concealed most of the structures. It was, however, easily removed by treatment with weak hydrochloric acid. On that half of the nodule which is labeled b, the layer was two millimeters thick and presented on its surface a cast of the ventral surface the impression of which was better visible on piece a, which was mostly free of the white deposit. The removal of the deposit exposed on piece b the entire dorsal surface and on piece a the third and fourth coxae and sternites. The photograph and the textfigures show the fossil as it now appears after cleaning. Only the first pair of legs is preserved. Total approximate length 19 mm. The carapace is 7.8 mms. long and 10.5 mms. wide. It is acutely pointed in front and its sides are convex. The posterior edge is evenly procurved and the lateral angles are slightly rounded. There is a very shallow, oval depression on the carapace, shown in Textfigure 5 by a dotted line. It represents what in life was a very low elevation on which a pair of perfectly flat eyes are situated. The eyes are oval, 0.44 mms. long, separated from each other by 0.68 mms. On account of their position on a low elevation they must have looked upward and outward. Two depressions are visible near the posterior edge of the carapace. In life they were tubercles separating the posterior edge into three even portions. The surface of the carapace is roughly punctate. Textfigure 5. Paratarbus carbonarius, n. sp., specimen No. 14864, Illinois State Museum. Holotype. Dorsal view. Anal operculum purposely omitted. Textfigure 6. Paratarbus carbonarius, n. sp., specimen No. 14864, Illinois State Museum. Holotype. Ventral view. The abdomen is evenly rounded behind. The anterior six tergites are well visible. They are abbreviated and divided by a low median crest. The first tergite is the shortest, especially in the middle and is the most procurved one. The sixth tergite is almost twice as long as the fifth and its intersegmental lines are straight. The lines between the posterior four tergites are very faint. The dorsal surface of the abdomen is roughly punctate. The ventral surface as represented by piece a, is shown in Textfigure 6. Sternum, all coxae and abdominal sternites are visible. The sternum is very long and narrow. The pedipalpal coxae are small and narrow, forming together a wedge between the first pedal coxae. The fourth coxae are by far the largest and their posterior edge forms an angle of ca.130°. All |