SILURIAN BRACHIOPODA 35 



Rhynchonella acinus, Hall, 1863 



Plate IV, Figs. 9-11 



, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 306, pi. 26, figs. 7-1 1. 



1879. 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 306, pi. 26, figs. 7-1 1. 1882. 



Were Rhynchonella acinus a rare species, it might readily be confounded 

 with the variety of R. indianensis, which bears but a single plication in 

 the ventral sinus. It appears, however, to have been very prolific, and its 

 abundance serves to emphasize its specific independence. The liability to 

 confuse it with any of the associated species arises only among forms of 

 immature growth. Beginning with a shell which is apparently in the actual 

 initial stage, measuring i.2x.8 mm., our series is very evenly consecutive 

 up to maturity, when the average dimensions are 8x6 mm. 



SPECIFIC CHARACTERS 



Mature Form (plate 4, figs. \\-\\b). Shell small, longitudinally 



ovate, subattenuate toward the beak, and truncate in front. Cardinal 

 margins long and rapidly sloping, extending more than half-way across 

 the shell ; sides flattened, slightly excavate. Valves subequally convex. 



Ventral valve full and rotund on the umbonal region, flattened at 

 about the middle, thenceforward sinuate ; beak mcurved, but not pro- 

 cumbent ; foramen generally concealed, or when slightly exposed, elongate 

 or subtriangular. 



Dorsal valve more flattened in the umbonal region and in the middle, 

 whence a low fold proceeds to the margin. 



Surface marked by low rounded plications. The ventral sinus 

 bears a single plication which is generally faint, oftQn nearly obsolete. On 

 each side of the sinus are four plications, those abutting on the cardinal 

 margins being indistinct. On the dorsal valve, the low, flattened fold bears 

 two plications which are the strongest upon the shell ; these are accom- 

 panied by three plications on each latus, making the whole number on this 



