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 strise on the 
inner laminae; in rare instances there are radiating lines on the exterior. 
Type, Atrypa. 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. Triplecia, Streptis 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 Triplecia a much more compact and homogenous division, comprising 
such species as Atrypa 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 Triplecia, though the dental lamellae 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 Triplecia, 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? Grayce, Davidson4 In Atrypa ( Tri- 
* The relations of this species are not well understood. The external form of the shell is that of T. 
extans, but in the specimens examined there appears to be a short median septum in the pedicle-valve, and 
no cardinal process in the brachial valve. It may prove necessary to separate this and the T. pi'imordialis 
of Whitfield, 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 acquainted with the internal characters of Triplecia extans, which at that date had 
not been made public, Mr. Meek withdrew this generic term. 
I See Silurian Supplement, pp. 144-147, pi. viii, figs. 24-32. 
