OF SOUTHERN INDIA. I47 



In Venus we notice the posterior cardinal (proper) in both yalyes ; in the left one 

 the two anterior cardinals form one, which is always thick and often grooved or nearly 

 bifid ; in the right we find the two anterior cardinals again joined to one, often bifid, 

 directed anteriorly and forming the postero-inferior margin of the pit ; the superior 

 margin of the latter is raised into a small compressed tooth. However, this 

 analogy in the arrangement of the hinge-teeth is not always so well seen ; in the sub- 

 genus Cryptogramma the hinge-teeth are more uniform, elongated, and diverging, 

 distinctly indicating a relation to the form of the hinge of the tapesinm. At the 

 same time the explanation of the hinge-teeth of Venus and Cytherea, as given above, 

 appears to me not only to facilitate the understanding of the relation which exists 

 between the two large generic groups of shells, but they also clearly point out the 

 character of the sub-family and the marked difference between it and the tapesin^. 



Dr. Romer has, as already stated, published several papers on the classification 

 and specific determination of the vENEEiN^in the " Malaco-zoologische Btetter" for 

 1866 and 1867, but as his large Monograph on this portion of '' Venus'' has not yet 

 appeared, I shall again refer to the various generic and sub-generic groups only so 

 far as they are important for us in a palseontological point of view, especially in 

 connection with the study of the cretaceous fauna, for the species belonging to it 

 have really been treated in a remarkably superficial manner. 



There are among the recent forms of vbneuinm, strictly speaking, only three 

 which deserve to be separated generically, Venus, Cytherea, and Circe, and even the 

 last two are, as regards the number, form, and description of the hinge-teeth, almost 

 identical. I do not know a single instance among recent shells where it would be 

 particularly difficult in distinguishing Venus from Cytherea, and the same may be 

 said as regards fossils ; the difficulty only rests with the preservation of the specimens. 

 Those forms, for instance, which Homes (Eossil. Moll, des Tert. Beckens von 

 Wien, vol. ii,) considers as intermediate between the two genera, and which he refers 

 to Venus, properly belong to Caryatis, a sub-genus of Cythereau 



6. Venus, Linn., 1758. Shell oblong, posteriorly .truncate, or narrowly 

 attenuated, solid, inequilateral, the anterior side being the shorter one, hinge with 

 three cardinal teeth in each valve, two primary and one lunular in the left,, 

 and one primary and two lunular surrounding a pit in the right valve; ligament 

 external, long, generally in a deep groove, pallial sinus small or sometimes 

 almost obsolete ; outer surface of shell variously ribbed, lamellated, striated, 

 or smooth. The name Venus has been restricted for the concentrically laminated, 

 rather tumid species, like V, Listeri, Gray, and others. Several sub-genera can be 

 distinguished, but they possess, as I shall show, no permanent characters which 

 would entitle them to rank as genera, equivalent to those noticed among 

 the TAPESiNAS, for in the present case the changes and transitions from one form 

 into the other are really so gradual and insignificant that it is impossible to depend 

 upon them. Eeeve (Conch. Icon., vol. xiv,) has published a Monograph of the genus. 



6a. Mercenaria, Schuhm., 1817. Shell roundly ovate, inflated, smooth, 

 or with partially obsolete concentric striae ; hinge-teeth rather close together and 



