Report of the State Geologist. 575 



of various depth, arranged either in quincunx (T. terminaUs) or 

 in radiating rows; in the latter case they may be distant from 

 one another without intervening ridges I T. umbonata), or lie in 

 radiating furrows, when they are either circular l T. miUeptmctata) 

 or subrectangular I T. OUaw 



Shell-substance composed of an outer calcareous layer with a 

 series of inner corneous lamellae. The outer layer varies in thick- 

 ness in different species, and is coarsely punctated by the pittingjs 

 constituting the surface ornamentation. The corneous layers are 

 impunctate. 



Type. Trematis termmalis, Emmons (sp.). 



Distribution, Lower (to upper?) Silurian. 



The 'species are largely American and mainly confined to the 

 faunas of the Trenton and Hudson .River groups. 



Schizomania, Mall and Whitfield. ls;:>. 



(Plate 6, Figs. 9-13.) 



Shells sub-circular in outline, inequivalve, unarticulated. 

 Pedicle-valve flat or concave; apex subcentral. A deep trian- 

 gular notch extends from just behind the beak to the margin, 

 where its arc is equal to about one-sixth of the periphery. The a pex 

 of this broad pedicle-notch is occupied by a triangular transverse 

 plate or listrium varying in size with the age of the shell, but 

 extending for one-fourth to one-third the length of the opening. 

 Surface marked by concentric growth-lines. On the interior no 

 muscular impressions are visible. Brachial valve more or less 

 convex, with the beak marginal. External surface radiately 

 striated. The interior bears a pair of strong posterior adductor 

 scars, lying close together in the unbonal region ; their outline is 

 elongate-ovate, indicating a progressive increase in size, and they 

 frequently appear to be divisible into anterior and posterior 

 elements. In front of them, at about the center of the valve, 

 are the small and faint anterior adductor impressions. A low 

 median ridge extends from the apex to beyond the center of the 

 valve. External surface marked by elevated stria? radiating from 

 the beak. 



Substance of the shell composed of perlaceous calcareous 

 lamina? which constitute the most of the shell. The inner layers 

 appear to be corneous. All are impunctate ('?). 



127 



