BRACHIOPODA. 867 



Trematis. 1 



Genus TREMATIS, Sharpe. 



1847. Trematis, SIIA.BPE. Quarterly Journal Geological Society, vol. Iv, p. 66. 

 1892. Trematis, HALL. Palaeontology of New York, vol. viil, pt. i, p. 138. 



Description: "Shell subcircular or transversely oval in outline. Pedicle valve 

 unevenly convex, more or less depressed over the posterior region; apex at or behind 

 the center; directly beneath it begins the pedicle-fissure, which transects the shell, 

 vertically widening to the posterior margin with straight or outwardly-curving 

 edges. Brachial valve evenly convex, with its apex marginal and slightly projecting. 

 On the interior, the pedicle-valve shows a faint median furrow extending from the 

 angle of the fissure to the apex of the shell; this groove widens at its apical termina- 

 tion and may represent a point of muscular attachment. The sides of the fissure are 

 often thickened by callosites similar to those sometimes seen in species of Orbiculo- 

 idea. From the apex of the valve extend radiating and branching vascular sinuses. 



"In the brachial valve the posterior margin is much thickened and broadly 

 grooved to allow the extension of the pedicle. This thickening does not take the 

 form of a cardinal area or shelf, but is rather a callosity closely appressed against 

 the interior surface of the shell, the central portion being projected beyond the 

 margin of the pedicle-valve. Directly below and in front of this area are two 

 transversely elongate scars, adjusters or posterior adductors, which are usually 

 partly concealed by the progressive overgrowth of the cardinal thickening. A faint 

 median septum begins between these scars and passes forward, becoming more 

 prominent over the tongue-shaped median elevation which separates the large 

 central scars. These impressions are oblique and are not simple, each appearing 

 to be composed of two, if not three distinct scars, making a posterior, a median and 

 an anterior pair. What appears to be the posterior pair is small, and sometimes 

 quite sharply defined, the central pair very much larger, and the anterior pair 

 narrow, situated at either side of the angle of the median callosity and separated 

 by the apex. The specialization of the first of these scars is not satisfactorily estab- 

 lished; the entire impression -is deeply excavated. In some well preserved speci- 

 mens there is also evidence of external marginal scars lying just in front of the 

 outer end of the posterior adductors. 



"Surface of both valves more or less completely covered by a beautiful orna- 

 mentation consisting of punctures or small pittings of varying depth, arranged 

 either in quincunx (T. terminalis) or in radiating rows; in the latter case they may 

 be distant from one another without intervening ridges (T. umbonata),or lie in radi- 

 ating furrows, when they are either circular (T. punctostriata) or subrectangular 

 (T. ottawensis). 



