i8g6. CLASSIFICATION OF THE PELECYPODA. 241 



just as Pelseneer, with whose scheme it naturally stands in contrast, 

 attacked the problem more from the general biologist's point of view; 

 and had it not been that they thus advanced from opposite sides, the 

 results obtained would probably have been more in accord, since both 

 naturalists are equally emphatic in stating that, while their terms are 

 borrowed from one particular feature, their classification is the result 

 of a consideration of the totality of characters. 



Dr. Dall's proposed classification was first shadowed forth in 1889 

 (7), in an article that deserved far more attention than it appears 

 to have received. He has now (8) revised and elaborated it, yet 

 retaining the general principles on which it was founded. 



He begins by dividing the Pelecypoda into three main orders, 

 which he considers proceeded more or less on parallel lines, viz. : — 

 I. Prionodesmacea. II. Anomalodesmacea. III. Teleodesmacea. 

 These he holds to be all descended from a prionodesmatic radical or 

 prototype, the first and second orders retaining more evident traces 

 of their origin than the third. 



This triple ramification from a common stock certainly seems to 

 be an advance on the more simple phylogenetic tree which satisfied 

 the Belgian malacologist. and more in accord with the fossil evidence. 

 At the same time, it is premature to assume that Pelseneer's tree can 

 only rank as an "intellectual weed," to quote Dr. Sollas's happy 

 characterisation of some such productions. Neumayr (5) started with 

 an order, Palaeoconchae, that comprised an assemblage of ancient fossil 

 forms of very various description, with which he apparently did not 

 exactly know what to do, and from which he derived his other 

 groups. Dall has no difficulty in showing that this group is nothing 

 more than a convenient cloak for ignorance ; nevertheless, although 

 he rightly avails himself of it as a matter of temporary convenience, 

 he gives it no place in his scheme. 



The Prionodesmacea correspond to the old section Asiphonida, 

 as given in Woodward's Manual (18), with the addition of Solemya, 

 and they thus include the oysters, scallops, arks, and all pearly shells. 



The Anomalodesmacea embrace the Anatinacea, Eusiphonacea 

 (or Clavigellidae), and Poromyacea (or Pelseneer's Septibranchia, plus 

 Verticordiidae and Euciroidae). 



The remaining forms are, of course, relegated to the third order, 

 Teleodesmacea. 



When the subdivisions of these three principal groups come to be 

 examined, much matter for criticism is revealed. 



The Prionodesmacea are first divided into (I.) those without teeth 

 in the hinge, and (II.) those with teeth, the latter being further 

 portioned out into : — 



A. Taxodonta (Nuculacea, Arcacea). 



B. Schizodonta (Pteriacea [=Aviculidae], Ostracea, Naiadacea, 



Trigoniacea). 



C. Isodonta (Pectinacea, Anomiacea). 



