28 i CRETACEOUS PELECYPODA 



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

 supporting the ligament may he sometimes taken for a part of the tooth, hut 

 t]iey do not, strictly speaking, heloug to it. The anterior muscular impression 

 is in MijtilicanUa usually the smaller one, while in Canlita it is larger than the 

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

 in hahitat from the former, heing commonly found in holes of rocks and corals, 

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

 foot with a distinct hyssus Avhich is wanting in Cardila. 



H. and A. Adams distinguisli three sub-genera under the names of Bcfjuina, 

 Bolten, Glans, Miihlf., and Tliecalla, H. and A. Adams. Of the first, M. semi- 

 orbictilata, 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 Cavdita 

 concamerala, Chem., has the interior margin peculiarly folded on itself. 



35. Cuvditamera, 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 I'adiately 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 raiocene beds of North America. Of recent species only few have 

 been described under the name of Lazarla. 



List of ceetaceops species. 



Opis (see Pictet and Campiche, Pal. Suisse, 4"<' ser., 3™« part., p. 3£7). 



1-16. — Opis Neocomiensis, Isara, Lorioli, Bulmensis, Mayori, ornata, Ilugardiana, Uneata, 

 Coqnandiana, elegans, Guerangeri, Ligerierisis, annoniensis, Truellei, hicornis,* and pusilla are enu- 

 merated by Pictet and Campiclie. 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 onlj- 

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

 which is sm Opisoma ; and the same is most probably the case with Truellei. — 0. galeata, d'Orb., 

 Cardium galeaium, Miiller, is a Trigonocmlia [^\^q postea, family Arcid.E ). 



17. — Op. Cetioma7ieHsis, Gueranger (Album pal. de la Sarthe, 1867, pi. 16, fig. 3). The 

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



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

 lOo-" livr., pp. 631 and 637). 



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

 is a veiy 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. Ilaleana, 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. CoU., No. 177, 186-1, p. 11,) questions their generic determination. 



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



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

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

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

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



