DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 327 



The only other species of Brachiopoda at this locality com- 

 monly thus overgrown and involved is Atrypa reticularis. 



Leptcena rJwmboidalis is cosmopolitan and has been dis- 

 cussed by many authors, who have shown its great variation 

 and wide distribution. So far as known, the youngest speci- 

 men heretofore figured is one represented by Mr. Davidson.* 

 This is an individual belonging to the third stage of develop- 

 ment, having a length of nearly 6 mm. and a distinct cir- 

 cular perforation of the beak. 



Orthothetes subplanus Conrad, 1842. 



(PLATE XVI, figures 14-20.) 



Streptorhynchus subplanum Hall. Twenty-eighth Ann. Kept. N. Y. State 



Mus. Nat. Hist., p. 151, pi. 21, figs. 26-33, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 288, pi. 21, 



figs. 26-33, 1882. 



The series selected to represent the development of this 

 species comprises fourteen specimens ranging from 1.5 mm. 

 to 26.5 mm. in length. The external features of form and 

 surface ornament are remarkably constant from the young to 

 the mature shells. There is, however, a slight progressive 

 modification in the relative convexity of the valves. The 

 dorsal valve of young and half-grown individuals is nearly 

 flat, while the ventral is moderately convex. In old speci- 

 mens both valves are convex, with the dorsal somewhat more 

 so than the ventral. The most marked changes due to 

 advancing growth are those which take place in the hinge. 

 Some mention of these is made under the description of 

 Strophonella striata, where it is stated that the pedicle-tube 

 retains its embryonic form and size nearly up to maturity, 

 after which it is obscured by the internal thickening of the 

 shell; also, that the callosity under the beak of the dorsal 



* British Fossil Brachiopoda, III, Devonian and Silurian, 283, 284, pi. 

 xxx, fig. 6. The same. General Summary to the British Fossil Brachiopoda, 

 289. 



