BRACHIOPODA. 139 



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 ap- 

 pearing 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 its apex. The specialization of the first of these 

 scars is not satisfactorily established; the entire impression is deeply excavated. 

 In some well preserved specimens, there is also evidence of external, mar- 

 ginal scars lying just in front of the outer ends 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 radiating furrows, when they are either circular {T. millepundata) or sub- 

 rectangular (T. Ottawensis). 



Shell-substance composed of an outer calcareous layer with a series of inner 

 corneous lamella. The outer layer varies in thickness in different species and 

 is coarsely punctated by the pittings constituting the surface ornamentation. 

 The corneous layers are impunctate. 



Type, Trematis terminalis, Emmons. 



Observations. The interior of the brachial valve in this genus presents at 

 first consideration a striking similarity to that of the pedicle-valve in Obolus. 

 In both the arrangement of the muscular scars is essentially into three pairs : 

 (1) the posterior adductors (cardinals in Obolus), (2) the anterior adductors, 

 (3) the externals. The second of these pairs is strongly excavated in both genera 

 and bordered by a median thickening. Herein lies the difference in both these 

 genera from Dinobolus, with which there is a superficial agreement ; these 



