EARLY DEVONIC HISTORY OF NEW YORK AND EASTERN' NORTH AMERICA 1 43 



ontol. N. Y. 1894. v. 8, pt 2, p. 280] from the cherts of Oriskany age on 

 Clear Creek, Union county, Illinois, but the illustrations of that shell are 

 meager and the specimens I have been able to secure for exact comparison 

 do not seem to justify the assumption of identity. 



Spirifer aroostookensis Clarke 



Plate 34, figures 6-r6 

 St'e p. iiq 



This species described from a single dorsal valve in the Chapman sand- 

 stone of Aroostook county, Maine [N. Y. State Mus. Bui. 107. p. 258. 1907, 

 and ante p. 1 19, pi. 20, fig. 5 ; pi. 30, fig. 5, 9], is characterized by its flat riblets, 

 slightly depressed or even grooved on top and all the surface covered by closely 

 concentric papillated lines. These riblets are 10-14 in number on each side 

 of the median fold, and the spaces or grooves between them are very narrow 

 and sharp with vertical sides. The only well known species with which one 

 might bring this shell into close comparison is Sp. concinnus Hall of 

 the Helderbergian and Dalhousie fauna. The differences however are clear : 

 The riblets of Sp. concinnus though low have not the broadly flat- 

 tened, depressed or even slightly grooved surfaces of S. aroostook- 

 ensis, nor the narrow vertical grooves between; the cardinal area is 

 higher and the beak more prominent and overarched ; as a rule the outline 

 of this shell is less extended on the hinge. The abundant and only observed 

 spirifer in the fauna at Highland Mills has all the distinguishing characters 

 of S . aroostookensis, occurring usually in the form of casts, interior 

 and exterior. The cast of the exterior presents with striking effect the 

 peculiarities of the flat riblets and the threadlike ridges representing the 

 dividing furrows. Often the ribs, on account of the more extended lateral 

 slopes will rise to 16-17 m number. The papillose surface is shown on well 

 preserved external casts only ; this is unlike that in S . cone i n n u s where 

 one observes it usually only on young shells, in maturity the surface pre- 

 senting a series of fine concentric lines. On the interior of these shells 

 there is considerable variation in the character of the muscle scars, those of 

 the ventral valve in old shells being deep set and somewhat expanded, often 

 with ramifying markings, but in younger individuals having less size and 

 prominence. No distinctive value can well be laid on such differences. 



Of all the specimens observed the mature ones reach about the pro- 

 portions of the original of S. aroostookensis, seldom attaining the 

 size or outline of the mature and prevailing S. concinnus in the 

 Lower Devonic limestones of New York and Dalhousie. Were it desirable 

 to enforce the distinctive traits of this species by contrast with its associates 

 in time it may be remarked that S. cyclopterus of the Helderberg 

 fauna is a fimbriate shell with rounded and sparse ribs, broad and sloping 



