284 CEETACEOUS PELECYPODA 



species in which the two laminae forming it were equally developed. The fulcra 

 supporting the ligament may be sometimes taken for a part of the tooth, but 

 they do not, strictly speaking, belong to it. The anterior muscular impression 

 is in Mytilicardia usually the smaller one, while in Cardita it is larger than the 

 posterior. Many of the species of the present genus (as restricted) also differ 

 in habitat from the former, being commonly found in holes of rocks and corals, 

 though not apparently perforating them voluntarily. The animal has a small 

 foot with a distinct byssus which is wanting in Cardita. 



H. and A. Adams distinguish three sub-genera under the names of JBeguina^ 

 Bolt en, Gkms, Miihlf., and Thecalia, H. and A. Adams. Of the first, M\ semi- 

 orhiculata, Linn., of the second, M. trapezia^ Linn., are to be considered as types; 

 the species differ slightly in form and habitat, and may be said to form conve- 

 nient sections of the genus. The last which has been proposed for Cardita 

 concamerata, Oh em., has the interior margin peculiarly folded on itself. 



35. Carditamera, Con., 1838, (Mioc. foss, p. 11; Lazaria, Gray, 1853, apud 

 H. and A. Adams). Shell elongated, moderately tumid, inequilateral, beaks 

 depressed, sub-anterior, surface strongly radiately ribbed ; hinge with two cardinal 

 and two lateral diverging teeth in each valve, the posterior teeth being in each case 

 much elongated, the anterior short and more or less pointed, sub-lunular. Type, 

 C. arata, from miocene beds of North America. Of recent species ouly few have 

 been described under the name of Lazaria, 



List of cretaceous species. 



Opis (see Pictet and Campiche, Pal. Suisse, 4™^ g^j.^^ 3me p^rL^ p^ g^T). 



1-16. — Opis NeocomiensiSj Isar(B, Lorioli, Bubisiensis, Mayori, ornata, Eugardiana, lineata, 

 Coquandia7ia, elegans, Guerangeri, Ligerie7isis, annoniensis, Truellei, bicornisj^ and pusilla are enu- 

 merated by Pictet and Campiche. The form of Opis is so characteristic that the genus cannot 

 be easily mistaken with another, but which of the species belong* to the genus Opisoma can only 

 be decided by the examination of the hinge. I have only seen tolerably good specimens of bicornis, 

 which is an Ojnsoma ; and the same is most probably the case with Tmellei.— O. galeata, d'Orb., 

 Cardium galeatum, Miiller, is a Trigonocoelia {YidiQ postea, family Arcid^). 



17.— Op. Cenomanefisisj Gueranger (Album pal. de la Sarthe, 1867, pi. 16, ^g. 3). The 

 figure is very indistinct, it is impossible to identify the species from it. 



18-19. — Op. megalodus and brevirostris are described by Eichwald from Russia (Leth. ross., 

 10^^ livr., pp. 634 and 637). 



20. — Op. bioculata, Kner, sp., (Cardita), Favre, Desc. Moll. foss. de Lemberg, 1869, p. 117, 

 is a very remarkable bilobate form, and it would be interesting to examine its hinge. 



•21-23. — Op, bicarinata and bella, Conrad, (Journ. Phil. Acad., new ser., iii, p. 327,) and 

 Op. Ealeana, d'Orb., (Prod, ii, p. 238,) are the only species as yet recorded from America; they are 

 all very insufficiently characterized, and Meek in his Check-list of cretaceous fossils (Smiths. 

 Misc. Coll., No. 177, 1864, p. 11,) questions their generic determination. 



24. — Op. widata. Con., is quoted from Palestine. 



* This species of Geinitz is identical with d'Orbigny's O. Galliennei (Pal. fran?. cret., pi. 257 bis, fig. 5,) which 

 is not mentioned in the text, nor in the Prodrome. O. Truellei, which d'Orhigny (Prod.) and Pictet identify 

 with bicornis, differs by being less high and having the ventral margin produced, convex, while in bicornis it is 

 truncate. (See also Grueranger in Album Pal. de la Sarthe, 1867, pi. 22, fig. 13.) 



