or SOUTHERN INDIA. 207 



a. Suh.family,—CARDnNm. 



In the arrangement of genera and sub-genera I shall principally follow 

 H, and A. Adams. 



1. Card'mm, Linn., 1758. This name has been reserved for a small group 

 of strongly costated species which Romer (Mart, and Chem., 1. cit., 18GS, p. 13,) 

 proposed to call Troiokloccu'dium. 



Shell rather thin, inflated, and nearly equilateral, with a sub-rectilinear hinge 

 area ; sm-face ornamented with distant radiating spiny ribs ; two small cardinal 

 teeth in each valve, one or the other of the upper teeth sometimes become obsolete ; 

 lateral teeth lamelliform, posterior gape generally distinct, rarely narrow. The 

 type of this genus for which Linne's name has been restricted is Cardium 

 costatinn, L., and species belonging to it seem first to appear in the eocene rocks. 

 The form is tolerably common during the miocene time, while only two species 

 are found living, fcostatum and hians). Some fossil species like Stu'digaUnum 

 distinctly indicate a transition to the next group. 



' la. Acantliocardium,* Gray, 1853. Shell tumid, rather solid, slightly in- 

 equilateral, with closely set, more or less, spiny or tuberculated radiating ribs ; 

 cardinal teeth small, lateral strong, posterior gape very narrow, or not at all 

 developed. The type of this section is C. aculeatum. Species belonging to it are 

 already found during the Jurassic period and gradually increase in number up to 

 the present date. Species like C. NuttalUi are again truly intermediate between 

 this and the next section, and should perhaps rather be referred to the latter, as 

 pointed out by H. and A. Adams. 



lb. Tectunciilus, A dans., 1757, includes the species of the type of C. asiaticum. 

 There is no distinction in the form of this and the previous sub-genus, only that in 

 the latter the ribs are usually more numerous and less spiny than in the former. 



Ic. Tmchycardkim, Morch, 1852 (?). This sub-generic group is distin- 

 guished by a thick, solid, and high shell ornamented with numerous radiating rough 

 ribs ; the hinge area is strongly curved and very short, the laterals being close to 

 the cardinals, and both strongly developed : in the right valve the two cardinals 

 are often grown together into one broad obliquely compressed tooth deeply grooved 

 on either side. 



The first fossil species of this type appear to be as yet known only from creta- 

 ceous rocks, and even in the subsequent formations they are scarce. Romer, 

 however, refers seventeen recent ones to it, C. elongatitm, Brug., being probably one 

 of the best known to concliologists. 



Gabb (in Pal. Calif., ii, p. 266,) proposes to call a number of cretaceous species, 

 of which C. j^i'odiictum, Sow., is the best known, Gninocardium ; the species 

 forming the sub-genus are said to be characterised by the intermediate ribs 



* Idem, Romer, 1868, 1. cit., p. 17. I do not tMnk that a change in the inflection of a word can in thi.s case be 

 considered as an alteration of the word itself. The original word heing Cardium, it should in the compound words 

 remain and not be changed to -cardia, because it then excludes the free use of the plural oi-cardium, and in any case 

 Ls liable to cause confusion. 



