or SOUTHERN INDIA. 59 



In the pursuit of palreontological researches there is scarcely any other family 

 of Peleeypoda so important as this, heing represented throughout the strata from the 

 time of the oldest sedimentary deposits upwards. The species living at the present 

 time may be said to be only the remnants of the group ; they are distrilmted all 

 over the world, but they are nowhere very nvimerous, and some of them belong to 

 the rarest yet known shells. Their maximvim of development appears to have been 

 during the Jurassic period ; at least there are scarcely any Jurassic fossilifcrous beds 

 known in various parts of the world, where about one-half (or a proportion very 

 near to it) of all the MYACEA does not belong to this family. On this account 

 I have especially endeavoured to give as complete a review of all the known 

 types as was possible for mo to do, and as far as I felt justified in giving an 

 opinion about them. Much still remains to be done as regards the comparison of 

 recent and fossil generic forms, for the latter are unfortunately mostly known in 

 the form of casts, both valves being often found closed, and thus generally making 

 an examination of the cartilage and other processes of the hinge almost an impossi- 

 bility. But even where the shells are known, the greatest diflB.culty is often felt in 

 exposing the hinge, and consequently the determination had to be principally based 

 upon the external form and the general characters of the shell. We can only 

 expect to get very gradually out of this chaos of so-called generic forms, 

 because whenever an author has an opportunity of studying one species properly 

 he prefers calling it a ' new genus,' rather than identifying it with another which 

 may be the same, but of which the internal characters are unknown; as has 

 been the case in several instances. 



The extent which I here give to the family is somewhat great, but it seems 

 impossible to separate the various forms satisfactorily. They include, like the 

 Tholadibm or Myid.e, one characteristic type of shells, and as regards the form 

 of the animals the variations are quite similar to those which I have repeatedly 

 pointed out in speaking of the other families. To make use, however, of the 

 distinctions indicated by the various forms of the shell, I have accepted here three 

 sub-families, the pandoris.e, tbraciin^, and anatinin^, the types of these sub- 

 divisions being Pandora, Thmcia, Aiiatina. Comparing the present family and 

 the Myidm, I may mention that the pandorin^, with their short siphons and fringed 

 orifices, as also the inequality of the shells, appear to be analogous to the Corhulce 

 and their allied genera, the tsraciinje by their more elongated siphons (and 

 posteriorly narrowed and produced shell) to the Necera, and the anatininai, by their 

 long united siphons and posterior large gape of the shells, to the mtiix^; thus 

 representing a variation which seems to repeat itself in almost every family of the 

 MYACEA. For these reasons I believe it to be more correct to retain closely 

 allied types of shells in one family, than to separate them into several, because if 

 we admitted a family division here, we would have to follow it in the other cases 

 also, which seems for the present hardly advisable. H. and A. Adams have included 

 in the Anatinidm several forms, like Poromija, Thetis, Necera, Theora, and Cham- 

 ostrea, which do not appear to belong to this family. The first two must be removed 



