270 PALEONTOLOGY OF NEW YORK. 



bifurcated, the distal extremity of each branch bearing a single deep groove. 

 This process is supported on a subrostral callosity, which also bears two short 

 spiniform crural points at its base. Shell-substance fibrous, impunctate(?). 

 Surface with obscure concentric growth-lines, and fine radiating sti'ias on the 

 inner laminae ; in rare instances there are radiating lines on the exterior. 

 Type, Alrypa extans, Emmons. Trenton limestone. 



Observations. The species of this genus appear to be subject to consider- 

 able variation, and there still remains some doubt as to the proper limitation 

 of the group. Triplegia, Stkeptis and Mimulus are terms which, with a little 

 latitude, one might regard as of equivalent value. Adhering, however, pretty 

 strictly to the type of structure exemplified in Triplecia extans, we shall 

 have for Triplegia a much more compact and homogenous division, comprising 

 such species as Atnjpa extans, Emmons ; A. cuspidata, Hall ; A. nucleus, Hall ; 

 Orthis insularis von Eichwald ; Triplesia Wenlockensis, Davidson, and perhaps 

 Camarella calcifera, Billings,* and some others. 



Triplecia Ortoni, Meek, the type-species of this author's genus, DicRANiscus.f 

 is a large, ponderous shell, retaining the subtrihedral form, and essentially the 

 internal characters of Triplegia, though the dental lamellse are obscured by 

 complication with the thick testaceous deposit usually found in the umbonal 

 cavities of both valves. There is apparently no good reason for not regard- 

 ing T. Ortoni as strictly congeneric with T. extans. 



Mr. Davidson has referred to Triplegia, with some doubt, a number of species 

 whose precise affinities have yet to be demonstrated. Of these are Atrypa? 

 apiculata, Salter ; Triplesia ? Maccoyana, Davidson ; Atrypa ? incerta, Davidson ; 

 Producta monilifera, McCoy, and Triplesia? Graya, Davidson.^ In Atrypa {Tri- 



* The relations of this species are not well understood. The external form rf the shell is that of T. 

 extaTis, but in the specimens examined there appeal's to he a short median septum in the pedicle-\alve, and 

 no cardinal process in the brachial valve. It may prove necessai'y to separate this and the T. prhnordiaUs 

 of Whitfibld, from this genus. The T. lateralis, Whitfield, from the Fort Cassin beds (Calciferous sand- 

 stone), contains a spoon-shaped process in each valve, that in the pedicle-valve being supported by a med- 

 ian septum. It therefore becomes necessary to remove this form to a distinct genus, and to a different 

 association and it will be described and illustrated in its proper place under the name Syntrophia. 



f After becoming acciuainted with the infernal characters of "Priplecta extans, which at that date had 

 not been made public. Mi-. Mkek withdrew this generic term. 



X See Silurian Supplement, pp. 144-147, iil. viii, figs. 24-82. 



