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 biloculare, Linne {= Gypidia or Pmtamerus 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 Paeastrophia 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- 



