Nautiloidea, and the amount of folding increases from the first septum to the later ones. Considerable attention has been paid to the form of the suture-lines, and their common course is as follows (fig. 107):-A lobe, the External lobe (or "ventral lobe"), occurs on the convex side of the shell, and is sometimes divided into two by a small saddle (fig. 113). On either side of this lobe comes a saddle (the External saddle); then a lobe (the first lateral lobe); then the first lateral saddle, the second lateral lobe, the second lateral saddle, and perhaps still further lobes and saddles, which are styled auxiliary. On the concave side of the whorl, where the two halves of the suture-line again meet, there occurs an unpaired Internal lobe. The last auxiliary saddle, occurring just above this lobe, is sometimes called the Internal saddle. The siphuncle of the ammonoids, with the exception of the one genus Clymenia, runs along the convex side of the shell. The group of the Ammonites is prosiphonate.* The surface of the shell is often, and particularly in the later types, highly ornamented with ribs and knots, which are independent of the suture-lines (figs. 113 and 114), the latter being visible only upon worn specimens and casts. times, as in Gault specimens from many localities, the inner and thicker nacreous layer is alone preserved, and the whole surface of the shell has a brilliant pearly iridescence. At el. Ill 211. 21s. a il. Fig. 107.-Suture-line of Finally, the bodies known as aptychi (fig. 105) or anaptychi are found associated with so large a number of ammonoid genera a further point of difference between this Nautiloidea. that they form group and the * Forms of Ammonite occur in which the earlier whorls are retrosiphonate; in one or two septa following on these the septal neck projects on both sides; and finally the shell becomes purely prosiphonate. But the broad classification of the Ammonoidea by the direction of the septal neck in adult forms seems well founded, since the older genera are so persistently retrosiphonate, while those of Mesozoic times are prosiphonate. Sub-group 1-RETROSIPHONATE SHELLS WITH SIPHUNCLE ON THE CONCAVE SIDE. Clymenia (fig. 108).-Shell spiral, broadly umbilicated; crosssection of whorl oval, flattened laterally. Mouth notched on either side. Suture-lines usually with only one lateral lobe, which is simply curved or angular. Siphuncle in contact with the concave side of the whorl. Retrosiphonate. Surface smooth; rarely ribbed. Exclusively Devonian. Sub-group 2-RETROSIPHONATE SHELLS WITH SIPHUNCLE ON THE CONVEX SIDE.-This sub-group includes all the shells known as Goniatites. The shells are very variously coiled, being at times involute, at times widely umbilicated. Cross-section of whorl may be flattened laterally, or broad (fig. 109), or fairly oval. The whole shell is thus sometimes discoidal, or sometimes almost globular through the strong convexity and the involute arrangement of its whorls. The suture-lines are occasionally slightly folded, but are commonly zigzag or bent rather sharply, the lobes and saddles being sometimes numerous. The lobes and saddles are, however, not subdivided by notched or foliaceous boundaries, as occurs in the next sub-group (fig. 112). The siphuncle is in contact with the convex side of the whorl, and is best seen where it emerges through any septum which may be displayed terminally on the specimen. Retrosiphonate. Aptychi have been recorded. Glyphioceras (fig. 109).-Shell involute, generally globose. Suture-line with external lobe divided by a small median saddle; external saddle narrow; lateral lobe pointed; lateral saddle broad and rounded. Surface practically smooth. (Examples:-Glyphioceras crenistria, Gl. sphæricum, Gl. truncatum.) Carboniferous and Permian. Gastrioceras.-Shell with wide umbilicus. Suture-line with broad and deep external lobe, with small median saddle; first lateral lobe deep and angular; second lateral lobe small and angular. Surface with longitudinal striæ; often with transverse ribs in addition, which are nodose near inner ends. (Example:Gastrioceras Listeri.) Carboniferous and Permian. Prolecanites.-Shell with wide umbilicus; whorls flattened laterally. Suture-line with several deep lobes and saddles, the lobes broadly pointed at the ends, the saddles rounded at the ends and narrowed near their bases. Surface smooth. (Example:-Prolecanites compressus.) Devonian and Carboniferous. Note.-Bactrites (Ordovician to Carboniferous) is the straight form of the Goniatites. Pronorites (Permo-Carboniferous), with its accessory serrations at the ends of the lobes, and Medlicottia (also Permo-Carboniferous), with its still more elaborate suture-lines, link the Goniatites completely to the sub-group of the Ammonites. Sub-Group 3-PROSIPHONATE SHELLS WITH SIPHUncle on the CONVEX SIDE.- -These are the successors of the Goniatites, and are distinguished, apart from the character of their septal necks, by a greater complexity in the suture-lines, the main lobes and saddles being variously subdivided and broken up (fig. 112). The mouth-border is produced, not notched, on the outer side. The surface of the shell is also inore strongly ornamented than in the preceding sub-groups, and is, indeed, very rarely smooth. This sub-group covers the great series of shells which are commonly styled Ammonites. While this world-famous name, given to all the "cornua Ammonis," must always remain a most valuable term in preliminary observations and for geologists in the field, it has become necessary to subdivide the old genus, and to establish a large number of new ones, each example of which may be properly styled an "Ammonite." Fischer's restriction of "Ammonites" to the members of the newer genus Arietites seems liable to cause confusion, and would destroy the utility of the word "Ammonite," which now, as formerly, covers a great series of shells allied to one another. New subdivisions are being, however, continually introduced, and for details larger and special works must be consulted. Ceratites.-Shell umbilicated; cross-section of whorl somewhat flattened laterally. Suture-lines sometimes with auxiliary lobes and saddles. The saddles are always rounded, the curve approaching semicircular; but the lobes are subdivided, their posterior border being zigzag* (fig. 110). Surface marked with ribs, which do not pass on to the outer border, but which often bear knobs as they approach it. Exclusively Triassic. Trachyceras.-Shell with rather narrow umbilicus. Sutures like Ceratites in the earliest species; but in later forms both the saddles and the lobes are denticulated (ie., bent into a zigzag form). Surface ribbed transversely, the ribs set with knots; a furrow runs along the convex margin. (Example:Trachyceras Aon.) Trias. Arcestes (fig. 111).-Shell involute, sometimes with small A very similar type of suture recurs among Cretaceous Ammonites referred to the family of the Amaltheidæ. umbilicus; whorls markedly convex. Mouth slightly reduced by the folding over of its outer and lateral borders. Bodychamber occupying more than a whole whorl. Sutures with numerous auxiliary lobes and saddles, the line being complex and foliaceous, so that the markings look like the outlines of little branching trees. The axis of each lobe and saddle is, however, straight. Surface smooth, or with fine transverse striations. The mouth-border appears to have become thickened internally at various stages of growth, so that casts (fig. 111) exhibit well marked and rather wavy grooves running at wide intervals from the outer to the inner side of the whorl. (Example:-Arcestes subumbilicatus.) Trias. Also Permo-Carboniferous of India, with another genus of Ammonite, Cyclolobus. Monophyllites.-Shell rather flat and discoidal, with fairly wide umbilicus. The whorls enlarge rather rapidly, giving a high mouth. Suture-lines with numerous lobes and saddles, which are foliaceous; but each saddle terminates anteriorly in a Fig. 112.-a, First lateral saddle of Monophyllites Simonyi. b, First lateral saddles of two adjacent septa of Phylloceras heterophyllum, showing the extremely foliaceous character of the suture-lines. single oval leaf-like form, although its sides are subdivided into leaflets (fig. 112, a). Surface smooth, or with fine slightly curving striæ. (Example:-Monophyllites Simonyi.) Trias. Phylloceras. Shell rather flat; involute, or with very small umbilicus. Mouth high. Numerous lobes and saddles, steadily increasing in size outwards, the saddles being broken up into leaf-like foldings with rounded terminations, the lobes into sharper notches (fig. 112, b). Surface smooth, or simply striated; no keel. Here and there external or internal thickenings of the mouth-border may be formed, producing in the former case ridges |