no. 5 



RHYNCHONELLOID BRACHIOPODS — COOPER 



23 



Cryptopora. An external difference between the two, on the other 

 hand, is suggested by Davidson's report on the exterior of Mannia of 

 "concentric scaly lines of growth, from which scattered adpressed 

 spinules seem to arise." The specimen figured by Davidson is very 

 small. Its measurements are given in lines: length 2 lines = 4 mm., 

 width 1^ lines = 3 mm. 



The pedicle valve interior is not well known, but Davidson (1874b, 

 P- X S7) speaks of a "narrow vertical plate" dividing the larger por- 

 tion of the beak into two parts. However, no indication of a median 



Fig. 1. — Partial side views of the brachial valves of A, Cryptopora, ca. X 8, 

 and B, Mannia, ca. X 16, showing maniculifer crura. 



septum can be seen in the beak region in Davidson's figure 10a, plate 7. 

 Inside the brachial valve the cardinal process is medial and the crura 

 are long but the socket plates are small. In one figure the crura are 

 convergent ; in the other they are divergent, but in both they are similar 

 to the crura of Cryptopora. Davidson reports them as being broken, 

 which is probably the reason why they are illustrated as not flattened 

 and expanded distally. 



The median septum of the brachial valve is illustrated by Davidson 

 as like that of Cryptopora in being short and very high. Unlike Cryp- 

 topora, however, its distal extremity is embellished by "two small tri- 

 angular plates united posteriorly, separate and angular anteriorly." 

 These form a small spoonlike trough which was interpreted by Thom- 

 son (1927, p. 297) as a terebratelliform structure. Thomson goes on 

 to state that the shell of Mannia will ultimately prove to be punctate, 

 but Davidson said emphatically that it is impunctate and referred it 

 to the Rhynchonellidae. 



