ACROTRETID^. 645 



When describing this species in 1884 I was not aware that a false area was present on 

 some ventral valves of the type species of the genus, and that the supposed muscle scars were 

 the main vascular sinuses. This caused me to question the generic reference. 



The ventral vahj^e of Acrothele dichotoma has the same general form as A. subsidua (Wliite), 

 but it has a thicker shell, and, so far as known, a concentrically striated surface. 



Formation and locality. — Middle Cambiian: (58) Shaly limestone in upper beds of the Secret Canyon shale, 

 east side of New York and Secret canyons, Eureka district [Hague, 1892, Atlas], Eureka County, Nevada. 



Acrothele gamagei (Hobbs). 

 Plate LX, figures 5, 5a-c. 



Oholella gamagei Hobbs, 1899, Am. Geologist, vol. 23, No. 2, pp. 114-115, figured. (Described, figured, and discussed 

 as a new species.) 



Acrothele gamagei (Hobbs), Geabau, 1900, Occas. Papers Boston Soc. Nat. Hist., No. 4, vol. 1, pt. 3, pp. 615-617; fig. 

 48, p. 617; PI. XXXI, figs. la-d. (Described and discussed. Pig. 48 is copied from the.figure accompanying 

 the preceding reference.) 



Acrothele gamagei (Hobbs), Shimer, 1907, Am. Jour. Sci., 4th ser., vol. 24, pp. 176 and 177. (New locality mentioned.) 



Acrothele gamagei (Rohhs), GnABAxs Sind Shimer, 1907, North American Index Fossils, vol. 1, p. 200. (Character- 

 ized.) 



General form subcircular to transversely broad oval in outline, with the posterior margin 

 slightly transverse. Ventral valve slightly conical with the apex at about the posterior fifth 

 of the length of the valve ; the apex is formed of two elongate tubercles with a slight depression 

 between them; at the posterior end of the depression a small pedicle opening occurs; back of 

 the pedicle opening a more or less indistinctly defined false area extends to the posterior margin ; 

 the posterior margin of the valve arches slightly upward. 



The dorsal valve is gently convex with posterior portion curving slightly downward so as 

 to fit closely into the upward-curving margin of the ventral valve ; beak marginal. 



The surface is marked by narrow, concentric undulations of growth, a few obscurely defined, 

 radiating ribs and fine, concentric, irregular, undulating, elevated lines ; the latter have numerous 

 minute tubercles on them over the central portions of the shell, and sometimes on the outer 

 lines of the old shells; this results in a finely granulated surface that may be seen with the aid 

 of a strong lens. The concentric lines cross the false area with only slight irregularity. The 

 shell appears to be corneous and built up of several thin layers or lameUse. 



The interior of the ventral valve shows an elongate depression on each side of the apex, a 

 small visceral area, and not very strongly defmed main vascular sinuses that originate back of 

 the pedicle opening and curve forward into the body of the valve. The interior of the dorsal 

 valve has a low, rounded, median ridge that extends forward of the center where it bifurcates; 

 a narrow, sharp, median septum occurs on the posterior portion of the ridge ; the main vascular 

 sinuses originate near the posterior margin beneath the beak and extend obliquely forward 

 into the valve. 



The only muscle scars seen are the elongate, oval central scars of the dorsal valve, which 

 are situated on the slopes of the median ridge back of the transverse center of the valve. 



Observations. — As stated by Grabau [1900, p. 615] this is the Massachusetts representative 

 of Acrothele matthem (Hartt) of the St. John formation of New Brunswick. With the additional 

 information afforded by a collection made by William P. Rust for the United States National 

 Museum, the similarity between the two forms is still more striking; they have the same general 

 form, surface, and convexity; but A. gamagei attains a larger size and differs in the position of 

 its main vascular sinuses; the latter may be seen by comparing Plate LX, figures 5b and 5c, 

 with Plate XLI, figures Ic and Id. 



The specific name was given in honor of Miss A. O. Gamage, of South Bristol, Maine. 



Formation and locality. — Middle Cambrian : (326b [Grabau, 1900, p. 617]) Braintree slate, beloio high tide about 

 100 feet {30.5 m.) east of the quarry at East Braintree, Abington quadrangle ( U. S. Geol. Survey); and (5) siliceous shale on 

 Hayward Creek, Braintree, Dedham quadrangle (U. S. Geol. Survey); both in Norfolk County, Massachusetts. 



