On the General (.'lass'ijicatlon of tlie Pehrt/po'Ia. 



X. — A Discussion of the General Classification of the 

 Pelecypoda. By M. Colley Marc:h, M.Sc, Geological 

 Department, Manchester University. 



[Plate III.] 



The Pelecypoda, like all other animals possessing skeletons, 

 can be classified from the standpoint of their hard or soft 

 parts. The ideal method, where both are taken into con- 

 sideration, is only available to the paleontologist when the 

 soft parts leave some trace on the hard. 



In the case of the Lamellibranchs, the modern classifi- 

 cations are based on the gills or the teeth. Dall, writing in 

 snpport of a general hinge classification, said, that as the 

 gills leave no impression on the shell, a gill classification 

 must necessarily exclnde all fossil forms, and so do away 

 \\\i\\ the possibility of forming a phylogenetic classification. 



It might also be argncd that the evolution of gills is not 

 of sufficient taxouomic importance for the division of orders 

 and suborders. It is generally acceded that the pelecypod 

 gill is homogenetic, being evolved from a type in which 

 there was a main rachis giving off hollow and partially 

 flattened leaflets. This type is held to have been developed 

 in the earliest Pelecypoda. The object of specialization in 

 gills and gill-chambers is twofold, to secure the maximum 

 respiratory surface and a separation of the incoming and 

 outgoing currents. These ends have been secured in the 

 Pelecypoda by specialization along one line only, that is by 

 elongation of the leaflets and the upgrowth of the free ends. 

 The junction of these upturned ends has procured the 

 division of the respiratory chamber. Ridewood has shown 

 that in the connections between the opposite sides of the 

 leaflets ciliary junctions preceded organic. 



These stages in gill development, then, are of great 

 interest as showing the evolution of the gill, but are useless 

 taxonomically, for they were followed by all lines diverging 

 from the common ancestal stock. They form only trans- 

 verse divisions across the general classification and can no 

 more be used to subdivide the group than can the articulation 

 of the femur with two or three bones of the pelvis in the 

 Ichthyosauria and Plesiosauria be taken as breaking those 

 groups up into orders and suborders. 



Moreover, in general evolution the gills are singularly 

 unaffected by any change in environment or habit such as 



