270 PALAEONTOLOGY 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 striae on the 

 inner laminsB ; 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, Streptis and Mimulus are terms which, with a little 

 latitude, one might regard as of equivalent value. Adhering, however, pi-etty 

 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 calrifera, 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 lamellas 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 Atri/pa? 

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

 Producta monilifera, McCoy, and Triplesia? Graya, Davidson. J In Atrypa (Tri- 



* The relations of this species are not well understood. The external foi-m cf the shell is that of T. 

 extans, but in the specimens examined there .appears to be a short median septum in the pedicle-\'alve, and 

 no (-ardinal process in the brachial valve. It may prove necessary to separate this and the T. primordialis 

 of Whiti-'IELD, from this genus. The T. lateralis, Whitfield, fi'om the Fort C.assin 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 therefoi-e becomes necessary to remove this form to a distinct jjenus, and to a different 

 association and it will be descriV>ed and ilhisti-ated in its proper place under the name Stntrophia. 



1 Aftei- becoming- acquainted with the internal characters of Trljilccia extans, which at that d.ite had 

 not been maile public, Mr. Mkbk withdrew this generic tei-m. 



I See Silui-ian Supplement, pp. 114-147, pi. viii, tigs. 24-32. 



