i8 MEMOIRS OF THE STATE MUSEUM 



rare in the normal form as in O. clcganhila. Our representative series 

 of this species affords variations between the following limits of size : 

 .5 mm. length x .75 mm. width (minimum), and 17 mm. length x 20 mm. 

 width (maximum). 



* 



STROPHOMENA RHOMBOIDALIS, Wilckens, 1769 



PLATE II, FIGS. 1-13 



, Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. Hist., p. 151, pi. 22, figs. 



4-10. 1879. 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 288, pi. 22, figs. 4-10. 1882. 



This well known species, although extremely abundant in the mature 

 state, is correspondingly rare in its undeveloped condition. The young 

 specimens which have been found are nearly all more or less broken, and 

 it is evident that while young, the shell was thin and delicate, consequently 

 few of their remains have been preserved. The series which has been 

 selected is, however, very complete in its representation of the distinct 

 phases of growth through which the individuals pass in their development 

 from youth to maturity. The initial form, without radiating striae ; the 

 second phase, a shell radiatingly striate, without undulations ; the third 

 state, striated and concentrically undulated, but without the angular 

 geniculation of the valves in front ; and the last phase, with the full 

 form and characters of maturity, offer a series of changes, not often trace- 

 able in Silurian brachiopods. 



The development of the characters of the hinge-area is also very sat- 

 isfactorily demonstrated and affords some interesting points of comparison 

 with certain forms of Streptorhynchus and Strophonclla. These features 

 are noticed at the end of the description of the species, Strophonclla 

 striata. 



SPECIFIC CHARACTERS 



Mature Form (plate 2, figs. 4, 4^, 10, 13). Shell semi-elliptical or 



semicircular in outline. 



