100 PALEONTOLOGY OF NEW YORK. 



In A. coriacea Lixnarsson found small circular scars in the middle of this 

 valve, abutting closely against the median septum. The septal characters seen 

 in the two species mentioned are also shown in Walcott's figures of A. sub- 

 sidua, White,* Mr. Matthew has described some features of the hinge-line as 

 follows: "Just within the posterior margin [of the pedicle-valve] there are four 

 minute pits, of which the two inner correspond to small tooth-like projections 

 of the dorsal valve ; the two outer ones are opposite the posterior ends of the 

 comma-shaped grooves of the umbonal depression."! We are not confident 

 that we have been able to detect these characters satisfactorily on Mr. Mat- 

 thew's typical specimens, but the suggestion has already been made that with 

 fossils as liable to distortion and imperfect preservation as these frail bodies, 

 great caution is needed in the separation of permanent and fugitive features. 



The shell-substance in this genus is essentially corneous and, as observed by 

 LiNNARSSON, is composed of several lamin?e. The surface is usually devoid of 

 other ornamentation than concentric growth-lines, but in A. granulata, Linnars- 

 son, A. subsidua, White, and A. Matthewi, Hartt, the concentric lines are more 

 or less interrupted, producing a granular or papillate appearance. 



The known species of Acrothele are few, and all its American representa- 

 tives belong to primordial faunas. In 1874, Dr. White described, under the 

 name Acrotreta ? siibsidua,t a species from Antelope Springs, Utah, which he 

 subsequently (1880, loc. cit.) referred to Acrothele. Mr.WALCOTT, in 1884,§ 

 referred to this geims the Lingula Matthewi of Hartt, from the St. John group, || 

 and, in the same year, described the species Acrothele ? dichotoma,"^ from the 

 Prospect Mountain group of Nevada. This species, however, bears a very 

 distinct subapical slope, and was' subsequently transferred by its author to the 

 genus Acrotreta.** Of the two species described by Linnarsson, A. coriacea 

 and /(. gramdata, the latter has been reported by Dr. Davidson, from the Upper 



* Bulletin No. 30, United States Geological Survey, pi. ix, figs. 4 a, 4 c. 

 t Transactions of the Royal Society of Canada, p. 40. ISS.*). 



I Geofji'aphicul and Geolog-ical Exploration West of 100th Meridian ; Pi-elim. Rept. Invert. Foss., \t. 6. 

 § Bnlleliii No. 10, United Statf.s Geological Survey. 



II Mr. Matthkw has designated two varieties of this sjunries: (a) jirlm((, (h) lata. 

 *i\ PaliEontology Eureka District, loc. cit. 



** Bulletin No. 30, United States Geological Survey, p. 107. 



