226 CRETACEOUS PELECYPODA 



XIX. Family,— TBIDA CNID^, 



The animals are characterized by an entirely closed mantle with thickened 

 margins, the external fringes of which are free, the two siphonal openings are 

 separated and situated on the lower side, usually with the margins fringed, the pedal 

 opening is in front of the beaks corresponding to a more or less distinct opening of 

 the shell ; the foot is small and cylindrical ; mouth with long narrow lips, at the 

 end of each of which there is a pair of pointed labial palps; gills two on each side. 



The shells are usually equivalve, free, often enclosed in growing coral, solid, 

 sub-triangular, rather elongated, tumid, rarely smooth, mostly on the surface strongly 

 radiately ribbed and concentrically lamellar or striated; the beaks are incurved 

 and close together; the lunula is more or less opened and flattened, ligament 

 linear situated along the posterior margin of the shell ; the hinge consists of a few 

 teeth situated along the hinder margin ; muscular impression single, situated near 

 the middle of the inferior margin, pallial line entire. 



The three genera included in the present family are — 



1. Eurydesma, Morris, 1845, (Strzelecki, New South Wales, &c., p. 275, and 

 Dana in Geol. of Expl. Exped., p. 699). Shell oval or roundly cordate, rather 

 thin, but very much thickened near the beaks, concentrically striated or nearly 

 smooth ; beaks strongly incurved with a sort of an excavated and gaping lunette 

 in front ; ligament large, occupying the greater part of the posterior, more or less 

 straight hinge area, which is broad and extends below the beaks so as to make the 

 ligament almost internal, one large, sub-conical cardinal tooth in the right valve 

 somewhat curved upwards and corresponding to a pit in the left ; several small 

 muscular impressions near the hinge, but no other larger ones perceptible, neither 

 has the pallial impression been as yet traced out; type, K cordata, Sow. sp., from 

 the New South Wales palaeozoic rocks. 



Morris suggests that this very remarkable shell should be placed nediV Avicula, 

 but the general form, the excavated and gaping lunula, the strong development 

 of the ligament below the beak, and the presence of a strong cardinal tooth in the 

 right valve all tend towards a relation of the genus to Tridacna and Sippopus, 

 from which it diflPers by the want of external ornamentation. The relations to 

 Sippopus have been already alluded to by Dana. 



2. Tridacna, daCosta, 1776,* (Elements of Conch., pp. 274 and 298, pi. vii, 

 figs. 4-5). Pedal opening of the shell wide, hinge in the right valve with one 

 short cardinal tooth posterior to the beak and two remote lateral, in the left valve with 

 two small cardinal and one prominent lamelliform posterior lateral tooth ; ligament 

 below the beaks thickened, and often lodged in a special spoon-shaped excavation. 

 DaCosta, I find, was the first to use the name Tridacna for our shells in a 

 strictly generic sense, giving an excellent figure of what appears to be T Cumingii 

 or compressa, 



^ Klein used for some species of THc?«ma the generic name ChamcBtracJicea, and it is believed that his figure 

 represents a Tridacna; but as the figure as well as his characteristics are rather vague, the name has not come into 

 use at all, and it seems of no advantage to introduce it now. 



