OP SOUTHEEN INDIA. I37 



adopted a special family. The study of the very numerous fossil forms has, I think, 

 fairly established the propriety of this altered classification. H. and A. Adams' 

 Cyrunoideib^, proposed for the genus Cyrenoidea, Jonas, do not belong, 

 I believe, to this order, but to the LTIGINACEA. 



Looking upon the relations which the present order offers, as compared with 

 the previous one, we have in the family Fbtricolibm a good connecting link. They 

 mostly have the united mantle margins and a rather small somewhat digitiform 

 foot similar to that of the MY A OB A, while they partially possess the prolonged 

 divided siphons of the T ELLIN A CEA ; their burrowing habits recall 

 the PHOLADAGEA, but the general character of organisation of the animals 

 as to the division of the siphons, the form of the gills and of the palpi, and the 

 hinge of the shell, is that peculiar to the VENEBACEA in general. Similar 

 repetitions of the characters of previous orders are met with in other families. 

 Thus, for instance, do the bosiniin^ represent, strictly speaking, quite a peculiar 

 type of shells, combining various characters. Some of them have the siphons 

 elongated and united, like the Myibjs, but a thick crescent foot such as we only 

 find again in the Ericinidje ; the shells often have great affinities to the Tellinid^ 

 in the form and distribution of the hinge-teeth as well as in the form of the 

 pallial sinus. 



The Glossidje, in part, and also the CardiiBuE, appear to form in the arrangement 

 of the hinge-teeth transitions from the VENEBACEA to the CSAMACEA^ 

 In the former family the hinge-teeth are, namely, often placed parallel to the hinge 

 margin, which is one of the principal characters of Chama, while in the Carbiibm 

 the cardinal teeth cross each other, which does not occur in any other family ; but in 

 some of the more elongated species, particularly those living in fresh and brackish 

 waters ( Lymnocardium) the transmutations from the ordinary arrangement of the 

 teeth in the Veneribm can clearly be traced to that of the typical Qarbiibm, 

 Thus, on the whole, all the different genera appear to form a natural order belonging 

 to one type of shells. 



In a palseontological point of view the VENEBACEA are of very great 

 importance, and many species characteristic for different strata belong to this order. 

 When compared, however, with the MY A CEA, they may be said to belong to a more 

 recent period. The MycB and their allies seem to have their greatest development 

 in the lower mesozoic epoch, the CRAM A CEA in the middle and upper mesozoic, 

 the VENEBACEA in this last and in the lower cainozoic, the TELLINACEA 

 in the cainozoic and recent, and the PROLABACEA in the recent epoch. At 

 the same time forms belonging to the VENEBA CEA are not absent in older forma- 

 tions. Some species of Betricolibm, Glossibm, and probably also of the Carbub^ 

 are to be met with already in palaeozoic rocks, and from that time their number gradu- 

 ally increases until the present epoch. The Veneribm do not make their appearance 

 much before the cretaceous period, and the Cyrenib^ are found at the base 

 of that period in brackish and fresh-water deposits. 



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