68 CEETACEOUS PELECYPODA 



26. Homomya, Agassiz, 1843, (Etud. crit., p. 154). Shell like Myaoites,i\\m, 

 concentrically striated, but not punctated; hinge, according to Terquem (Mem. 

 Acad. Metz, 1855, [extr.] Observ. s. Etud, crit., &c., p. 35, pi. 1, figs. 3-5,) eden- 

 tulous, slightly emarginated below the beaks, and with prominent nymphs for the 

 attachment of an external ligament. Several conchologists altogether ignore the 

 genus Somomya, There is no doubt a great difficulty in distinguishing between 

 the former and the present genus, when the hinges of the specimens are not per- 

 fectly well preserved. But this does not entitle us to ignore what we already know ; 

 but should rather elicit further information on the subject. When externally 

 comparing Homomya and Fanopcea the distinction between the two becomes almost 

 impossible, because there is scarcely any difference to be traced between the form 

 of one and the other, except in the respective thickness of their shells. 



I could mention here a few palaeozoic genera, as, for instance, Sanguinolites, 

 Orthonota, Sedgwickia, and others, to which no doubt several species have been 

 referred which very likely belong to the Myibm, but the typical species are said to 

 be closely allied to Solemya which, in the organisation of the animal, is closely 

 allied to the Crassatbllidje and Astartib^e. 



27. Tyleria, H. and A. Adams, 1857, (Genera II, p. 368). Shell oblong, equi- 

 valve, very thin, gaping and sHghtly produced posteriorly. Hinge with an oblique 

 cartilage-pit in each valve, a calcareous lamina extending from the cartilage-pit ante- 

 riorly as far as the muscular scar; this ridge is parallel to the margin, with which 

 it is connected by short,jb2aiisvei'sc--s6ptav T\ fragilis is the only recent species 

 known, and its shell is nearly papyraceous ; it has some relation to Anatina, and also 

 to some of the forms named by A gas si z Mactromya^ and others. 



28. Anatina, Lamarck, 1809. Shell thin, translucent, sub-equivalve, atte- 

 nuated, and gaping posteriorly, beaks fissured, internally often with radiatino* ribs 

 cartilage situated in a spoon-shaped cardinal process in each valve, furnished in 

 front with a transverse ossicle, pallia! line with a wide and shallow sinus. 



Agassiz' fossil genus Flatymya (Etud. crit., 1843, p. vii,) must, at least in part, 

 be considered as identical with Anatina proper ; for PL dilatata, for instance, does 

 not exhibit any characteristic differences, and many others, (except rostrata. As*, 

 which I shall mention under Plectomya,) are based upon such indistinct casts that 

 they are even as species worthless for determination. If the form can be considered 

 of any importance, it may be possible to give Flatymya a sub-generic value, as some 

 of the species, referred to it by Agassiz, are much more compressed than recent 

 Anatince, 



29. Cercomya, Agassiz, 1843, (ibid. p. 143,) appears to be a characteristic 

 sub-genus or genus of Anatininm ; it is largely represented among secondary fossils ; 

 it differs from Anatina proper by having the posterior end much elongated and 

 narrow, and longitudinally grooved with a somewhat distinct curve in the stria.^ 

 of growth. This indicates that the siphons were not much prolonged beyond the 

 posterior end of the shell, being laterally rather compressed, and perhaps partially 

 separated, near their terminations. Thus in several particulars these shells would indi- 

 cate a relation to Lyonsia and Fandora, The recent representant of Cercomya 



