V0 1889 n, J PKOCEEDINGS OF THE NATIONAL MUSEUM. 243 



the whole membership of a species, say of the Veneridw, taking all 

 ages, above the larval stage, into account. 



We may now proceed to consider the groups of which these orders 

 should be made up. 



To the Anomalodcsmacea I refer the Anatinacea, the Myacea, the 

 JZn&iphonacea or Tubicolw, the Solenomyacea, and the Adesmacea. 



In the first three groups or suborders we have forms whose relation- 

 ship will hardly be questioned, embracing also some instances of the 

 most remarkable specialization of characters. To refer to a few, I may 

 mention Aspergillum, Clavagella, Cuspidaria, and Poromya, using these 

 names in their widest sense. 



From several characters of the gills and other soft parts paralleled 

 in the Nuculacea, Solenomya was at first affiliated by me with the 

 Prionodonts.* On mature consideration, while admitting that the last 

 word on this subject has not yet been put on record, I am inclined to 

 believe that this genus is an Auodont which has retained certain 

 archaic features of the soft parts, and represents in the Anomalodcsmacea 

 a survival analogous to that of the Nuculacea among the Prionodonts. 



From a very early period the Solenacea have been associated with the 

 forms now gathered in this order. Professor Verrill has called atten- 

 tion to the fact that Tagelus caribwus and its allies have the organiza- 

 tion of Tellinacea, and I have removed them to the vicinity of Psammobia, 

 in my Check-list of the Marine Shell-bearing Mollusks of the south- 

 eastern coast of the United States. (Bull. U. S. Nat. Mus., No. 37.) 

 But are the Solenidce to be left behind 1 After due consideration I can 

 see no sufficient reason for such a course, and conclude that the united 

 siphons and burrowing habit, with its resulting specialization, do not 

 warrant it. I have therefore excluded them. 



In the Adesmacea or Pholadacea we have the most remarkable 

 specialization of the hinge known in the whole class. The relations of 

 the parts are best understood by a study of the open-shelled forms like 

 Zirplma crispataov Bamea costata] and the young of the closed Pholads. 

 In the adidt forms of the latter, specialization has proceeded so far that 

 the true relations of the parts are more or less masked. In Bamea 

 costata we have the anterior dorsal, margin of the valves reflected 

 dorsally until the anterior adductors following the shell pass the axis 

 of motion of the hinge and pull at the short end of the lever, tending to 

 open the valves, instead of to close them. The posterior adductors pull 

 in the normal way and balance the anterior ones. The ligament is re- 

 duced to an ineffective film. The cartilage remains as a survival, but 

 reduced to such dimensions as to be practically of no use. Its elastic 

 properties are lost and it merely serves to connect two little processes, 

 the feeble remnants of the original fossettes. An appendage analogous 



*Bull. U. S. Nat. Mus., No. 37, p. 2(3, July, 1889. 



t See Proc. Acad. Nat. Sci. Philadelphia for 1889, pp. 274-76. 



