General Chissfjication of the Peleci/poda. ^'^ 



C. Grobbex. 



Grobben used the hinge in conjunction with other anatomiral 

 characters as the basis of his classification. He recognized three sub- 

 classes : — 



1. Protobranchia, equivalent to the Protobranchia of Pelseneer. 



2. Be.ijnodoiifa, equivalent to Neumeyer's order of that name. 



3. Ambonodonta : (1) Eutaxodonta (Arcidne). 



(2) Heterodonta (senfu Neumeyer). 



(3) Scliizodonta (setisu Neumeyer). 

 (■i) Anisomyariaus {se7isu Lamarck). 



D. Dall. 



Ball's orders are three in number : Prionodesmacea, Teleodesmacea, 

 and Anomalodesmacea. 



T/ie Prionodesmacea are described as having hinges -which " are the 

 product of evolution applied to the development of (among other things) 

 teeth to the hinge-margin, or of amorphous teeth" (14, p. 452). This, 

 as an isolated quotation, might seem to show that he considered the 

 transverse direction of the Prionodesmacean teeth to be secondary. 

 Quotations from his earlier paper of ]889 will, however, show that in 

 his conception this transverse direction of the teeth was primary. 



1. (13, p. 452.) " Attention has been already called to the fact that 



there can be but three fundamental types of hinge ; which may be 

 called anodout, prionodont, and orthodont, the latter term being 

 used to indicate the forms in which the cardinal margin has 

 become longitudinally plicate." 



2. (13, p. 447.) There are three fundamental types of hinge : — 



(1) The simple edentulous margin. [Anodont.] 



(2) The hinge in which the teeth are developed transverse to the 



cardinal margin. [Prionodont.] 



(3) The hinge in which the direction of the teeth is parallel to 



the margin. [Orthodont.] 



I am disposed to think that the time relations of the difiereut hin"-es 

 are those of the order in which I have cited them. 



The Teleodesmacea include those forms in which the prionodont and 

 orthodont types are combined, the latter being superimposed on the 

 former either by a fusion of the transverse teeth or by tlie subsequent 

 development of longitudinal teeth. 



The Anomalodesmacea contain those Pelecypoda in which the dorsal 

 margin is without a distinct hinge-plate, the armature of the hino-e 

 being " feeble, often obsolete, or absent." 



Ball's three orders, therefore, were made for tliose Lamellibranchs 

 which have teeth — 



(1 ) transverse to the hinge-margin ; 



(2) parallel to the hinge-margin ; 



(3) so degenerate as to show no definite aflinity to the other two 



orders. 



