BRACHIOPODA. 
233 
former probably representing the scar of the adductor, and the latter the im¬ 
pressions of the diductor muscles. In the brachial valve the beak is obtuse 
and closely incurved into the deltidial cavity or spondylium of the opposite 
valve. The dental sockets are long and narrow, their inner margins being 
bordered by two broad, convergent crural plates, which extend toward the bot¬ 
tom of the valve, but do not reach it. These sloping plates are supported by 
two vertical septa, with which they are united, not at their extremities, but 
obliquely, just within their free edges. At the anterior angles of these free 
edges, there are two long, straight or slightly curved, rod-like crural processes 
extending into the anterior cavity of the shell. Beneath the beak is a faintly 
developed, bilobate or multilobate cardinal process. The muscular scars lie on 
the surface of the valve between the two vertical septa, and extend for some 
distance in front of them. They are divided by a low axial ridge. 
Shell-substance fibrous, impunctate. 
Type, Conchidium hiloculare, Linne (= Gypidia or Pentamerus conchidium, Dal- 
man et al.). Upper Silurian limestone, Gotland. 
Observations. The great diversity of form presented by the fossils com¬ 
monly referred to the genus Pentamerus, necessitates a careful scrutiny of their 
structural relations. From this extensive group of species a number of forms 
deviating from the typical structure have been separated. In 1859, Mr. 
Billings brought together, under the name Stricklandia, one peculiar associa¬ 
tion of such forms. It has already been shown that certain small, equiconvex 
or reversed-convex pentameroid forms, as Parastrophia and Anastrophia should 
be held in close generic relations with Camarella The variations which 
occurred during the Devonian, at a period when the climacteric development 
of the pentameroids had passed, have been grouped under various designations, 
as Gypidula, Pentamerella, Amphigenia, etc., which will presently be considered 
in detail. Recognizing the distinctive value of these terms, there still remains 
for consideration a group of fossils which first appeared in the middle Silurian, 
rapidly attained an enormous development and probably disappeared in the 
middle Devonian; in other words, the genus, “Pentamerus.” This group, com- 
