SILURIAN BRACHIOPODA 23 



Streptorhynchus subplanum, Conrad, 1842 



Plate II, Figs. 14-20 



, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 151, pi. 31, figs. 26-33. 



1879- 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 288, pi. 21, figs. 26-33. 1882. 



The series selected to represent the development of this species com- 

 prises fourteen specimens ranging from 1.5 mm. to 26.5 mm. in length. The 

 external features of form and surface ornaments are remarkably constant 

 from the young to the mature shells. There is, however, a slight progres- 

 sive modification in the relative convexity of the valves. The dorsal valve 

 of young aod half-grown individuals is nearly flat, while the ventral is 

 moderately convex. In old specimens 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 men- 

 tion 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 valve uniformly 

 increases in size from the youngfest forms to full-grown specimens. 



SPECIFIC CHARACTERS 



Mature Form (plate 2, figs. 15, \<^a, 17, 20). Shell semicircular or 



semi-elliptical, depressed convex; hinge-line longer than the width of the 

 shell ; cardinal angles flat and extended. 



Dorsal valve moderately and uniformly convex except at the cardinal 

 angles ; umbo not defined ; beak small. 



Ventral valve convex on the umbo, less convex below, and in many 

 specimens the marginal portion is flat or slightly concave ; beak small, 

 somewhat arched. Hinge-area nearly equal in both valves, usually appear- 

 ing as a deep angular groove along the cardinal margin. Under the beak 

 of the dorsal valve is a large triangular callosity, grooved on the inside, 



