Doll — Hinge of Pelecypods and its Development. 457 



We may now proceed to consider the groups of which these 

 orders should be made up. 



To the Anomalodesmacea I refer the Anatinacea, the My- 

 acea, the Ensiphonacea or Tiibicolm, the ISolenomyacea and the 

 Adesmacea. 



In the first three groups or suborders we have forms whose 

 relationship will hardly be questioned, embracing also some 

 instances of the most remarkable specialization of characters. 

 To refer to a few I may mention Aspergillwn, ClavageUa, 

 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 belie ve that this genus is an 

 Anodont which has retained certain archaic features of the soft 

 parts and represents in the Anomalodesmacea a survival analo- 

 gous to that of the Nuculaoea among the Prionodonts. 



From a very early period the Solenacea have been associated 

 with the forms now gathered in this order. Prof. Yerrill has 

 called attention to the fact that Tagelus caribceus and its 

 allies have the organization of lellinacea, and I have removed 

 them to the vicinity of Psammobia, in my Check-list of the 

 Marine shell-bearing Mollusks of the Southeastern coast of the 

 United States.* But are the Solenidm to be left behind ? 

 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 

 liave therefore excluded them. 



In the Adesmacea or Pholadacea we have the most remarka- 

 ble 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 Zirphcea crispata or Barnea costata 

 and the young of the closed Pholads. In the adult forms of 

 the latter specialization has proceeded so far that the true rela- 

 tions of the parts are more or less masked. 



In Barnea 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 reduced 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 



* Bull. U. S. Nat. Mus , No. 37, 1889. 



