64 CEETACEOUS PELECYPOBA 



tertiary deposits ; those from cretaceous beds may with equal probability be referred 

 to the former genus. Of recent species about 25 are upon record. 



Several of the species of Thracia resemble in their rather solid and opaque shell 

 the M/C8, at least more so than any other genus of the Anatinidje, 



11. Ixartia, Leach, 1852, (Syn. Moll. Great Brit. ed. by Gray, p. 272 ; 

 Bupicola, El. d. BL, 1802, not Briss., 1769). This genus generally has a somewhat 

 irregular, sub-oval shell, the hinge with a vertical cartilage process, and the pallial 

 sinus small and obtuse. Lx. distorta, Montagu, is the best known species. 



lla. Chenu (Man. ii, p. 40) adds Ligula of Becluz as a sub-genus of 

 Thracia, while H. and A. Adams identify the same with Feriploma. The shell 

 figured by Chenu as Zig. declivis is characterized by a strong oblique cardinal 

 process in each valve supporting the cartilage, certainly very similar to that of 

 Feriploma, except that it is less twisted. In addition to this it is said to differ from 

 the last-named genus by being equivalve and very thin. The name Ligula having 

 been, however, by previous authors differently applied, cannot stand, and even if the 

 few distinctions of the shell could be considered sufficient to justify a separation 

 from Feriploma, I very much doubt that they will hold good in a comparison with 

 the next shell, unless they be based on the somewhat irregular form of the median 

 portion of the shell of the so-called L, declivis. 



12. Calcara, Becluz, 1868, (Bev. de Zool. No. 2, p. 53). Eorm typical of 

 the TRRACiiNM, nearly equilateral, equivalve, hinge with a spoon-shaped cartilage 

 process in each valve dh^ected anteriorly, pallial sinus deep. This name has been 

 proposed for the fossil Anatina ohlonga of Philippi (En. Moll. Sic. I, p. 8). 



13. Feriploma, Schuhmacher, 1817, {Galaxura, Leach, 1852). Shell ovate, 

 inequivalve, left valve being more ventricose than the right one; beaks fissured 

 internally, often supported by radiating ribs, hinge with a spoon^shaped oblique or 

 horizontal cartilage process in each valve, external ligament thin. P. incequivalvis, 

 Schuhm., is the type of the genus. 



13a. The shell named by Couthuoy Cochlodesma only differs by being sub- 

 pellucid and somewhat less inequivalve and inequilateral than other Feriploma. 

 Some species of the Jurassic Corimya of Agassiz, like Cor. tenuistriata, C. glabra, 

 and others, appear to be very closely related to this type of Feriploma. 



14. Felopia, H. Adams, 1868, (Proc. Zool. Soc, p. 16). Shell oval, inequivalve, 

 closed on both ends, surface scabrous ; hinge with a long horizontal excavated car- 

 tilage process ; ligament placed in a deep groove. Fel. hrevifrom (ibid. pi. iv, fig. 16,) 

 is the type of the genus, which differs from Feriplom^a by the want of an internal 

 rib below the cartilage process and by not having the beaks fissured. 



15. Alicia, Angas, 1867, (Proc. Zool. Soc, p. 908). Shell inequivalve, 

 resembling a small Thracia, but the posterior portion is much smaller than the 

 anterior, internally sub-nacreous ; beaks entire ; hinge composed of a posterior callus 

 in the right valve fitting in a cavity in the left one, and an anterior marginal tooth 

 or ridge ; cartilage internal under the umbones, covered by a large triangular ossicle ; 

 pallial line deeply sinuated. Two species, All. angustaia and elegantida, are described 

 from Port Jackson. 



