1S96. CLASSIFICATION OF THE PELECYPODA. 243 



the Dreissensiidae, which Pelseneer banished to the company of the 

 Unionidae, is justified by their embryonic history. 



The famiHes placed under the order Anomalodesmacea would, 

 anyhow, be near allies, and hence it is no great concern that Dall 

 classes the Septibranchia with the Anatinidae. It is an old quarrel ; 

 nevertheless the anatomists would seem to have the best of the argu- 

 ment, for the existence of the septum in the one group, be its morpho- 

 logical origin what it may, is surely sufficient justification for the 

 separation of the two. 



The third order, Teleodesmacea, practically corresponds to the 

 Eulamellibranchiata of Pelseneer : its subdivisions, five in number, 

 are, however, very different. Of these the Pantodonta (Allodesmidae), 

 the Cyclodonta (Cardiacea, Tridacnacea, and Isocardiacea), and the 

 Asthenodonta (Myacea and Adesmacea [=Pholadidae, etc.] ) call for 

 no comment. Nor do the Telodonta, beyond the remark that the 

 Solenacea, here included with Veneracea, Tellinacea, and Mactracea, 

 have hitherto usually been classed with the Myacea. The remaining 

 group, Diogenodonta, comprises Pelseneer's Submytilacea (minus, of 

 course, the Unionidas and their allies, ^theriidae, removed to the 

 Schizodonta, and Dreissensiidas, with Modiolopsidse referred to 

 Dysodonta) and the Chamacea with their allies the Rudistae. 



The latter surprising innovation has been introduced by Dr. Dall 

 on the ground that " Echinochama has a free nepionic stage, in which 

 it has the form, hinge, and other characters of Cardita.'" Boehm (17), 

 agreeing, he says, with Steinmann, considers, on the other hand, that 

 the prodissoconch in question most nearly approaches the group of 

 Astarte sUidevi. Granting that this nepionic stage proves the affinity 

 of the Chamacea to the Carditacea (or Astartacea), it is still doubtful 

 if this sanctions the wide separation of these two, and the Rudistae, 

 from their allied forms the Cyclodonta ; might it not equally be held 

 of sufficient force to warrant the inclusion of the Carditacea in the 

 Cyclodonta ? True, it would have spoilt the present definition of 

 that group ; but the assemblage itself would have been rendered no 

 more anomalous than is Schizodonta. Moreover, it is worthy of note 

 that the prodissoconch in Chama itself by no means resembles that of 

 Echinochama. Suppose it should turn out that, in spite of the strong 

 resemblance of the hinge-teeth in the adult forms, the nepionic stages 

 should reveal a different line of descent for each of these two genera ? 



Destructive criticism is of course easy ; but what alternative pro- 

 posal can be made when doctors differ thus ? Practically none, till 

 more is known of the anatomy and life-history of the majority instead 

 of the minority of the forms, and till our museums contain series 

 of specimens of all ages with properly dissected examples of the 

 animals, in lieu of the simple shell of two or three adult individuals 

 of each species (so-called). 



Dr. Dall himself freely admits his ' provisional classification,' as 

 he wisely is careful to call it, to be far from perfect, but maintains " that 



