296 Raymond — Common Devonian Brachiopods. 



Changes during Development : Convexity of Valves. — When 

 the individuals become about -1o mm in length the valves are 

 still nearly equally convex, and the ventral area is curved and 

 inclined backward, so that the beak of that valve projects 

 beyond the beak of the dorsal valve. In stages but little later, 

 when the shell is slightly over l mm in length, the ventral valve 

 is four or five times as deep as the dorsal, and the ventral area 

 becomes more erect, so that the beak is anterior to the hinge 

 line. 



Plications. — When the shell has reached a length of from 

 •45 to '60 mm a sinus is formed in the ventral valve and very 

 soon after its initiation a fold is produced in the opposite shell. 

 This stage, in which there is no other ornamentation than the 

 fold and sinus, continues for some time. The largest speci- 

 men showing this state is l*5 mm long and 2"26 mm wide. Shells 

 at this period are almost globular and difficult to separate from 

 the young of Ambocmlia umbonata unless carefully examined. 

 (See No. 1, Kow -6, Plate XV.) 



The plications are introduced in pairs on the lateral margins, 

 each pair coming in outside the older ones. 



Spirifer mucronatus Conrad. 

 (Plate XVI, Eows 1, 2.) 

 Pal. N. Y., iv, 1867, p. 216, pi. 34, figs. 1-32. 



Delthyris consobrinus d'Orbigny. 



Pal. N. Y., iv, 1867, p. 222, pi. 35, figs. 15-23. 



As the development of these two species is practically the 

 same, they will be considered together. 



Protequlum. — On the beak of a well- 



preserved dorsal valve of S. mucronatus 



--^7[XJ\^^~— is th e impression of the initial shell. It 



v^-^/ is circular, somewhat convex, with a 



-^ on o • v curved hin^e. The diameter is *ir nm 



Figure 20. — Spirifer „ & 



mucronatus Conrad ; out- (ngure ^o)« 



line of protegulum and JSfepionic Shell. — In the nepionic 



shell in nepionic stage ; gta e the ghell j g oyal in outline, and 



taken from a dorsal valve. ■, " •, ,-, 1 t-> , , -, 



x 28. broader than long. xJoth valves are 



convex, the ventral a little the deeper. 

 The hinge width is less than the greatest width below. The 

 surface is smooth, with no fold, sinus, or plications. 



Neanic Stages. — In the earliest neanic stages a sinus is 

 developed in the ventral valve, bounded by two strong ridges, 

 which are the first plications. Following this a fold is formed 

 in the dorsal valve and plications are added on the margins in 

 pairs, each pair coming in outside the older ones. The later 

 plications do not reach the beak. 



