150 CRETACEOUS PELECYPODA 



a sub-genus of Veiuis, and gives its characteristic thus: " canio valvce sinistrce 

 dentibus quatuor, quorum tribus apice convergentibus et approximatis, unico remoti- 

 uscule sub lunula ; valvce dextrce dentibus tribus dimricatis et approximatis, dente 

 lateral i fnunularij tiullo." In general character the species of Ci/therea are gener- 

 ally solid, smooth, or concentrically, rarely radiately, ribbed shells of an ovately 

 cordate, or trigonal, moderately tumid form ; the pallial impression is usually provided 

 with a sinus, though sometimes very small; occasionally it is nearly entire; the 

 cardinal teeth are often grooved or divided, but this varies greatly in the different 

 species; the inner edge of the shell is mostly smooth. Eomer distinguishes the 

 following sub-genera or sections, which with the exception of Meretrix we adopt 



here. 



8«. Cytlierea, Lam., 1S05, f Meretrix apud Romer). Shell triangularly 

 cordate, generally smooth, sometimes concentrically sulcated ; inner edge smooth, 

 pallial sinus very small or sometimes nearly obsolete. Type V. meretrix, Linn. 

 Romer enumerates 14 species. 



8&. Ticela, Link., 1807, (Trigona, Meg. v. Miihlf., Trigonella and TFachydesma, 

 Conrad). Shell triangular, with smooth surface exhibiting only striae of growth; 

 ligament strong ; ventral margin slightly convex, internally smooth, pallial sinus 

 moderate, but always distinct, extending horizontally and rounded at the end. 

 Romer (loc. cit.) describes 33 species Avhich by Reeve are mostly referred to 

 Cijtherea. Cyth. [^TiceUi] crassatelloides. Con., is probably the best known shell in 

 collections. 



8c. Callista, Poll, 1791, includes a number of cordately or triangularly ovate 

 species, moderately compressed, smooth or concentrically sulcated ; ligament slightly 

 prominent; pallial sinus deep, sinuated and rather broad, at the end often abruptly 

 pointed. Cyth. \^CalUsta'] erycina, Linn., is probably the best known form of this 

 sub-genus. Romer describes 33 recent species. 



The shell of Cyth. {Call.'] gigantea, Chem., could probably form a special sub- 

 genu.s with its allies. The form is peculiarly elongated, and so is also the posterior 

 cardinal tooth, while the two others are small. We have a species of this type 

 already in our cretaceous rocks. 



8(/. Aphrodina, Conrad, 18G8, (Am. Journ. Conch., iv, p. 216). "Shell 

 rounded or sub -oval, striated or sulcated* ; hinge in the left valve with three diverging 

 cardinal teeth, the anterior as thick as the middle one, or thicker, and a straight, 

 compressed, transversally rugose lateral tooth parallel with the margin above it ; 

 pallial sinus deep, and similar to that in Caryatis, Romer. The type of this 

 genus is Meretrix Tqopana, Conrad, of which we have one valve only which shows 

 the hinge, and that is nearly as perfect as if the valve were a recent specimen." 



who proposed it. It is, as I had already stated, to be greatly regretted that authors do not themselves retain the names 

 which they have introduced in science, but the oases are different where an author omits a name purposely, or out of 

 ignorance of the work of others, and where an author makes a change with the object of improving or coiTecting the 

 nomenclature. Discretion in such cases is necessary. I have also previously stated my reasons for the superfluous intro- 

 duction of the name Meretrix in a sub-generic sense. We must not introduce three n.ames when they are not required. 

 * This variation ia the characters evidently is in antioipation of any further species or specimens being discovered ! ! 



