42 MEMOIRS OF THE STATE MUSEUM 



subsequent addition of plications takes place slowly and from the cardinal 

 margins. 



Rhynchonella indianensis, Hall, 1863 



Plate III, Figs. 17-38 



, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 163, pi. 26, figs. 12-22. 



1879. 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 306, pi. 26, figs. 12-22; pi. 27, figs. 



4-6. 1882. 



Rhynchonella mdianensis is, beyond a doubt, the most prolific species in 

 the rich fauna of the Waldron beds, and by virtue of this fact, we have been 

 enabled to ascertain the developmental phases through illustrative series of 

 exceptional completeness. It is noteworthy that the mature shell of this 

 Rhynchonella presents variations from the adult type, which are so great that 

 in a certain sense they might be regarded as passing the limitations of 

 specific identity ; however, the general form and expression of the shell are 

 characteristic, so that, in spite of these variations, no confusion with allied 

 species of the same fauna can arise, nor need there be any hesitation to 

 assign to the different forms a varietal significance only. Probably ten 

 thousand individuals of this species have passed under our observation, and 

 of this large number, fully one-half have been immature forms. 



SPECIFIC CHARACTERS 



Normal Mature Form, containing two plications in the sinus of the 

 ventral valve (plate 3, fig. 21). Shell subtriangular or broadly 



ovate ; length nearly equal to, sometimes slightly exceeding the width. 

 Umbo prominent, subacute; cardinal slopes extending one-half the length 

 of the shell, and flattened. 



Ventral valve depressed convex, rounded at the beak ; apex pointed 

 and slightly incurved, exposing beneath it the elongate, narrow foramen 

 and the inconspicuous deltidial plates. Dorsum for the first one-third the 

 length of the shell rounded, thence, anteriorly, gradually becoming de- 



