General Clatsfjicalion of the Peleci/podd. 9!^ 



C. Grobben. 



Grobben used the hinge in conjunction with other anatomical 

 characters as the basis of his classihcatiou. lie recognized three sub- 

 classes : — 



1. Protobi-anchia, equivalent to the Protobranchia of Pelsoneer. 



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



3. Ambonodonta : (1) Eutaxodonta (Arcid;e). 



(2) Ileterodonta {seiuu Neunieyer). 



(3) Schizodonta {sensu Neunieyer). 



(4) Auisomyariaus [sensu Lamarck). 



D. Dall. 



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

 and Anomalodesmacea. 



The Prionodesmacea ^xe described as having hing-es which " are the 

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

 teeth to the hinge-niai-gin, 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 secondavv. 

 Quotations from his earlier paper of 1889 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 anodont, prionodout, and orthodont, the latter term being 

 used to indicate the forms in which the cardinal margin has 

 become longitudinal!}' 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. [Pricnodont.] 



(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 different hinges 

 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 tlie 

 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 hinge 

 being " feeble, often obsolete, or absent." 



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

 which have teeth — 



(1 ) transverse to the hinge-margin ; 



(2) parallel to the hinge-margin ; 



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



orders. 



