5. Lithotrya truncata. [Pl. IX], [fig. 1.]

Anatifa truncata. Quoy et Gaimard. Voyage de l’Astrolabe, Pl. xciii, figs. 12 to 15, 1834.

L. scutis in profundam tergorum plicam insertis: carinæ cristâ centrali prominente et rotundatâ in parte superiore: rostro et lateribus rudimentalibus, carinæ latitudinis quindecimam fere partem æquantibus.

Scuta locked into a deep fold in the terga: carina with a prominent central rounded ridge in the upper part: rostrum and latera rudimentary, about 1/15th of the width of the carina.

Mandibles, with nearly three times as many pectinations between the first and second teeth, as between the second and third teeth; distance between the tips of the first and second teeth equal to that between the tip of the second tooth and inferior angle. Maxillæ widely notched, with the inferior part forming two prominences. Caudal appendages shorter than, or barely exceeding in length, the pedicels of the sixth cirrus.

Friendly Archipelago, Mus. Paris; Philippine Archipelago, Mus. Cuming; imbedded in coral rock.

Capitulum rather thick, with the five main valves having their free apices, diverging and truncated. The upper and old layers of shell do not here scale off so readily as in many of the foregoing species; and hence an unusually large proportional length of each valve projects freely above the sack; and the valves are of unusual thickness. The capitulum is very nearly as wide at its summit as at its base, owing to the divergence of the apices of the valves. The scuta and terga are articulated together by a conspicuous fold, which, when seen from vertically above, ([Pl. IX], [fig. 1 a´,]) appears like a deep wedge-formed notch in the terga. On the exterior surfaces of the valves, the teeth on the successive rims are approximate; on the inner surfaces, the rims are covered by strong yellow membrane, which is generally fringed with small horny spines.

Scuta, exterior surface convex, sub-triangular, with the apex truncated: seen vertically from above, there is a small rectangular indentation or fold which receives the projecting scutal margin of the terga. The inner growing or corium-covered surface ([fig. 1 b, b´]) is triangular, with its tergal margin largely hollowed out. Along the occludent margin there is a slight ridge, which terminates at the rostral angle, in both the right and left-hand valves, in a rounded, knob-like, roughened tooth. The lower part of the tergal margin is slightly inflected and roughened, where it meets the corresponding lower part of the scutal margin of the terga. There is a deep pit for the adductor muscle. The interior surface of the valve above this pit is faintly-coloured purple. The inner surfaces of both scuta and terga, are roughened with little points.

Terga, seen externally, are almost quadrilateral (owing to the apex being truncated), with the free margin facing the scutum, arched. Seen vertically from above, each shows a deep fold, which receives the lower part of the tergal margin of the scutum. In the foregoing species, a prominent ridge runs down the exterior surface of the terga from the apex to the basal angle, against which ridge, the margin of the overlapping scuta abuts: here this ridge, instead of projecting straight out, is oblique or folded over, and thus forms a furrow, receiving the margin of the scuta. The interior growing surface of the tergum ([fig. 1 b´, c]), presents so irregular a figure, that it can hardly be described; in area it quite equals the scuta; it is slightly concave; at the upper point of the carinal margin, there is a large, rounded, protuberant, roughened knob, which corresponds with a small knob on each side of the inner face of the carina; these knobs seem firmly united together by membrane. The scutal margin of the terga, in the upper part, forms a shoulder, largely projecting over the scuta; on its lower part, there is a small roughened projection. The occludent margin is arched and protuberant, with a slight fold above the knob on the carinal margin, just mentioned: this fold is caused by the protuberance of the central internal ridge of the carina, but is so small, that when the capitulum is seen from vertically above, it can hardly be distinguished. Finally, the basal half of the carinal margin, runs in the same line with the basal margin of the scuta.

Carina, moderately large; seen externally, the surface presents an elongated triangle, with the apex truncated; on the internal face ([fig. 1 b´, d]) of the free part, there is (instead of being concave as is usual) a great central ridge, which projects between the diverging apices of the terga, as may be seen from vertically above; hence the thickness of the upper part of the carina, in a longitudinal plane, almost equals its breadth. The edge of this ridge is rounded. The inner or growing surface of the carina is tinted purple, and lies in a plane, oblique to the longer axis of the valve; it is triangular, with the apex cut off, and the basal margin rounded and protuberant; it is not concave. There is a central raised line or slight ridge on this inner surface, and on each side in the upper part there is a small, white, roughened knob, corresponding with the similar knobs on the carinal margins of the terga.