posterior lateral teeth run obliquely from under the umbo towards the centre of the large posterior ear.

Ordovician to Carboniferous. Mostly Devonian.

Posidonomya (fig. 81).-Equivalve. Shell thin and laterally compressed. Obliquely and rather broadly oval. Umbos not prominent; hinge-line short and straight, without true ears. Concentrically marked. No teeth. See Estheria (Phyllopoda). Ordovician to Jurassic. Especially Carboniferous.

Monotis. Equivalve; typically small; form much like Posidonomya, but with a small rounded anterior ear and a more marked posterior one. Radially marked. No teeth.

Trias.

Daonella.-Ally of Monotis. Form rather semicircular, with a fairly long straight hinge-line. No ears. Umbos almost central and not prominent. Radially marked. No teeth. Trias.

Inoceramus (fig. 82).- Somewhat inequivalve. Varying much in size, some species measuring two feet or more across. Form obliquely oval, with a straight hinge-line. Umbos fairly prominent, sometimes twisted, and set well forward. Concentrically and boldly furrowed; rarely radially furrowed. The hinge-border bears, when viewed from within, numerous closely-set cartilage

boniferous).

Fig. 81.-Posidonomya Becheri (Car- Fig. 82.-Inoceramus Cuvieri (Senonian). Right valve. pits, which lie transversely to the length of the hinge. No teeth. Compare Perna below.

The outer layer of the shell is commonly well preserved in the species from the Chalk, though not in those from the Gault; it is easily recognised, even in fragments, by its fibrous crossfracture (see p. 324) and is often 5 mm. thick. The inner surface of these fragments is seen to be smooth and slightly undulating.

A sub-genus Actinoceramus (fig. 83) has been established for the singular species with deep radial furrows; but transitional forms occur.

Fig. 83.-Inoceramus (Actinoceramus) sulcatus (Gault).

Fig. 84.-Perna Mulleti (Atherfield Clay). Showing (a) the series of ligament-pits in the hinge-line, which is in this species exceptionally produced.

Mesozoic. Especially Cretaceous. Perna (fig. 84).-Sometimes markedly inequivalve. General resemblance to Inoceramus. Sometimes elongated posteriorly, but often approximately quadrilateral, with rounded ventral border. The umbo, which is acute, is set at the anterior end of the straight hinge-line. Concentrically marked. Numerous transverse ligamental pits. Toothless. Material of shell foliaceous. Trias to Recent.

Gervillia.-Inequivalve; resembling a much elongated Avicula. Hinge-line straight, with a very small anterior and a larger posterior ear. Umbo terminal, like a mere anterior rounding of the hinge-line. Concentrically marked. Oblique ridge-like teeth, running posteriorly; in sub-genus Hoernesia one cardinal tooth in each valve. Cartilage-pits conspicuous, broad, and set at some distance from one another.

Trias to Eocene. Typically a Mesozoic genus.

Pinna.-Shell thin, elongated, each valve triangular in form; gaping behind, so that the whole shell is wedge-shaped. Hingeline long and straight; umbos terminal. Marked with fine concentric lines. No teeth.

The inner shell-layer is thin, and is composed of aragonite; the prismatic (calcite) layer is thus particularly prominent. In the allied genus Pinnigena or Trichites (of Jurassic and Lower Cretaceous age) the prismatic fragments are found an inch or more in thickness.

Devonian to Recent. Especially Cretaceous.

Mytilus. The common marine mussel.

Shell rather thin;

elongated and approaching triangular, pointed in front, rounded behind. Smooth, or concentrically (rarely radially) marked. Hinge-line straight, umbos terminal. Sometimes one or two obscure cardinal teeth.

The modern mussels live near the shore-line, becoming uncovered at low water, and are attached, often to one another, by a coarse byssus. The inner shell layer is aragonite.

Trias to Recent.

Modiola (fig. 85).-Bears some resemblance to Mytilus, but in form approaches an elongated rectangle; the posterior end is more rounded than the anterior, and a broad fold often runs obliquely from the umbo to the base of the posterior margin. Umbos not quite terminal; hence this region of the shell has none of the triangular appearance so characteristic of the anterior end of Mytilus. Concentrically (rarely radially) marked. No teeth.

The modern Modiola is burrowing in habit, or forms a nest around it of fragments of sand, shells, &c.

Devonian to Recent. Especially Jurassic.

Lithodomus. - Close ally of Modiola. Shell cylindrical, narrowed behind, not greatly elongated. No teeth.

Burrows into stones (as at the famous Temple of Serapis), corals, &c., forming crypts which yield club-shaped casts (see p. 324

Carboniferous to Recent. Note.-For Dreissena, an ally of the above series, see p. 335.

Hippopodium (fig. 86).-Shell very thick and massive; form somewhatlike Modiola. Lunule

(Lias).

trically furrowed. One thick oblique cardinal tooth in each valve. The muscular impressions cause the genus to be here placed with the Heteromyaria; but S. P. Woodward regarded Hippopodium as "a ponderous form of Cypricardia or Cardita." Lias.

F. MONOMYARIAN ASIPHONATE FORMS (INTEGRIPALLIATE).

The shell is closed in the adult by one adductor muscle, which leaves a nearly central impression, placed rather towards the posterior side. It is always the anterior muscle that has disappeared. Except where specially mentioned, the members of this sub-group are toothless in the adult condition. Shell often inequivalve, and commonly attached by one or other valve. Hence, when loosened, the lower valve may reveal an outer scar of very various form, sometimes representing, as an external mould, another shell on which the young animal had become fixed. Occasionally in the Ostreide the young shell lies on some surface with prominent markings, such as that of Trigonia or Cidaris, and both valves become folded to suit the curves of the support. As growth proceeds, the nacreous layer is constantly being added to within, while the shell is also spreading at the margins; thus the original portions bearing the impress of the support become separated by new material, and form strangely marked umbos to the shell. The impressions are thus convex on the upper valve, concave on the lower; while within no trace of them is to be seen.

Ostrea (fig. 87).-Attached by left valve. Shell rather inequivalve, composed of foliaceous layers; often thick, especially near the umbos. Form rather flat, lower valve more convex. Irregularly rounded at ventral margin, more acute at dorsal, the umbos being nearly central on the hinge-line. Umbo of left

(lower) valve more prominent than that of right. Concentrically marked, with sometimes broad irregular radial foldings. A wellmarked triangular cartilage-pit occurs below the umbo. Shell composed of calcite (Sorby).

Trias to Recent. Doubtfully Carboniferous.

Alectryonia (fig. 88). A genus cut off from Ostrea to include forms with bold angular ribs and furrows in both valves, the

margins becoming consequently acutely folded, and the space occupied by the animal being much restricted in volume.

Trias to Recent; especially Cretaceous.

Gryphæa-Free, or attached only by umbo of left valve. Shell inequivalve, thick, oyster-like. Nearly equilateral. Left valve strongly convex, with umbo bent over and inwards; right valve smaller, flatter, or even concave, and sometimes reduced to the appearance of an operculum. The form of the shell varies considerably, being, like Ostrea, sometimes expanded and approaching circular, sometimes much narrowed. Concentrically marked.

Lias to Recent; especially Jurassic to Lower Cretaceous. Exogyra (fig. 89).-Much like Gryphæa, but both umbos twisted backwards almost spirally. Fixed by left valve, which is the larger.

Jurassic to Cretaceous.

Lima. Free, or attached only by byssus. Shell thin, equivalve; obliquely oval, slightly convex, gaping at anterior border. Umbos somewhat acute, ap proaching a central position, and separated from one another by a space in

which a groove occurs for the ligament, Fig. 90.-Hinge of Lima, which is partly external, partly internal (fig. 90). Short straight hinge-line, with a small ear on each side of umbos.

The following have been divided off as sub-genera :