BRACHIOPODA. 
139 
transversely elongate scars, adjustors 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. millepunctata ) or sub- 
rectangular [T. Ottawensis ). 
Shell-substance composed of an outer calcareous layer with a series of inner 
corneous lamellae. 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. ^ 5- ee . \~k. 
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 
