OF SOUTHERN INDIA. 143 



Considering geological history, the Veneracea are not very old. The first 

 forms externally very much resembling Cytherece, Callist(c, and Caryates are met 

 with in Jurassic (very seldom any in triassic) strata ; Er'rphyla (of the bosihhinm) 

 and perhaps Cytherea appear to be, however, the only genera well ascertained 

 to occur in the lower and middle mesozoic beds. In the cretaceous period the 

 number of species gradually increases, and many of the recent genera there make 

 their firet appearance. Their then geographical distribution, (as at present 

 known), very slightly, however, indicates the one they now possess. — In the older 

 tertiaries, forms of Cytherea, especially of Caryatis and Callista, are common, 

 but very few of Tivela, and scarcely any of Circe, Lioconeha, Venus, Siinetta, &c., 

 are to be found. On the whole, the character of the fauna is very little altered 

 in the eocene, as compared with that of the upper cretaceous period. In the 

 upper tertiaries, (the miocene, &c.), there is, however, a rapid change in the 

 fauna, and, with few exceptions, the forms are the same as those living at the 

 present day ; several of the recent tropical genera and some identical species 

 were then living in more northern European latitudes, indicating the then existing 

 conditions of the climate and its gradual changes. I look upon the family Veneeid^ 

 as particularly interesting in this respect, and the first and great desideratum we 

 have at present to look to is the exact study of the present habits of these aged forms, 

 in order to be able to make correct conclusions as regards the conditions of climate 

 and life during the middle and upper cainozoic period in Europe. 



I shall now give a short account of the generic and sub-generic forms of the 

 Veneeid^, according to their arrangement in four sub-families, — tapesina;, 

 VESERiNM, suNETTiN^, and DosiNiiN^ ; the first connects the family with the 

 Petricolid.e, and the last indicates a good passage to the Cyfsinidjs. A review 

 of the species described from cretaceous deposits will follow the general account. 



a. Sub-/am!ly,—TAPi:SlX.K 



The species referred to this sub-family agree with each other in several important 

 points of their structure. The animals have a digitiform foot, gradually attenuat- 

 ing towards the end, which is pointed ; near the lower base the byssus is generally 

 well developed ; the mantle margins are only for a short distance united below, but 

 sometimes separated at the outer edges all round; the siphons are moderately 

 produced, thick, united at the base, and for a short distance separated at the 

 end, with strongly fringed margins at the orifices. 



The shells exhibit a great deal of variation in their form ; they are always 

 inequilateral, the anterior side being the shorter one ; their surface is either smooth, 

 or more often concentrically elegantly ribbed, or sometimes radiately and divaricately 

 striated ; they are generally thin, and the inner margin is very rarely crenulated ; the 

 pallial sinus is of moderate size, either horizontal or ascending ; the hinge is composed 

 of three cardinal teeth in each valve ; the distinctive characteristic of the teeth 

 is, that they are always situated close together, as if radiating from one point; they 



