OF- SOUTIIERX IXDIA. I37 



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

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

 Cyrenoideibje, proposed for the genus Cyrenoidea, Jonas, do not belono-, 

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



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

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

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

 foot similar to that of the 3IYACEA, while they iiartially possess the prolonged 

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

 the PHOLADACEA, 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 dosiniin.e represent, strictly speaking, quite a peculiar 

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

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

 find again in the Ericinid^e ; the shells often have great affinities to the Tellinid.e 

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

 pallial sinus. 



The Glossid^, in part, and also the Cardiida;, appear to form in the arrangement 

 of the hinge-teeth transitions from the VENEBACEA to the CHAMACEA. 

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

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

 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 fLymnocardiuin) the transmutations from the ordinary arrangement of the 

 t^eth in the Veneridm can clearly be traced to that of the typical Cardiid^. 

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

 to one type of shells. 



In a palteontological 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 MYACEA, they maybe said to belong to a more 

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

 in the lower mesozoic epoch, the CRA3IACEA in the middle and upper mesozoic, 

 the VENEBA CEA in this last and in the lower cainozoic, the TELLINA CEA 

 in the cainozoic and recent, and the PUOLADA CEA in the recent epoch. At 

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

 tions. Some species of Tetricolidje, Glossidm, and probably also of the Cardudm 

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

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

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

 of that period in brackish and fresh-water deposits. 



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