NO. 1192. TRENTON FAUNA OF BAFFIN LAND— SCBUCHEBT. 159 



The form of B.minnesotensis is quite variable and can not be of much 

 aid in distinguisbing' this sbell from R. inwquivalvis. The two are often 

 found associated in the same beds, and it is then diflBcult to identify 

 them correctly. This fact led Winchell and Schuchert, in 1893, to regard 

 both as belonging to one species, E. incequivalvis. 



Locality and formation. — In the Lowville and particularly in the 

 Black Eiver stages of the Trenton in Minnesota and Wisconsin, Lex- 

 ington, Kentucky, and in the Cincinnatian group at Savannah, Illinois. 

 This species and B. incequivalvis suhtrigonalis are unknown at Silli- 

 man's Fossil Mount. 



RHYNCHOTREMA IN^QUIVALVIS (Castelnau). 



Spirifer inwquivalvis Castelnau, Essai Systeme Sil. I'Ameriqiie Septentrionale, 



1843, p. 40, pi. XIV, fig. 8. 

 Atrypa invrebescens Hall, Pal. N. Y., I, 1847, p. 146, pi. xxxiii, figs. 13a-13h 



(jirobably not p. 289, pi. Lxxix, fig. 6). 

 Bhynchotrema ina'quiraJvis Wjnciiell and Schuchert (part), Geol. Minn., Ill, 



Pt. 1, 1893, p. 459, pi. xxxiv, figs. 12-14, 24, 25 (not figs. 9-11, 15-23=7i;. 



minnesotensis). 



This species was first described by Castelnau, who obtained his spec- 

 imens from the "niagnesian limestone" of Drummonds Island. His 

 figures show the characteristic concentric lamellae which are almost 

 always present on these shells comiug from the Trenton, and this 

 feature distinguishes them from those found in the Lowville and 

 Black River stages. Hall subsequently described this shell as Atrypa 

 increhescens, but it is now generally known as Rhynclionella increbescens. 

 The species is a characteristic form of the Trenton, while in the Cin- 

 cinnatian group its descendant R. ca/pax attains large size, often great 

 rotundity, with marked concentric lamellae. The lamellar develoi)- 

 ment finds its extreme in R. perlameJlosvni. 



R. incequivalvis is abundant aL Silliman's Fossil Mount, the concen- 

 tric lamellae being also strongly developed in the specimens found. 



Collectors. — J. K. Carpender, A. H. White, and A. V. Shaw. Cat. 

 No. 28155, U.S.N.M. A number of specimens collected by Mr. R. W. 

 Porter are in the American Museum of Natural History. 



RHYNCHOTREMA INiEQUIVALVIS SUBTRIGONALIS (Hall). 



Airypa suhtrigonalis Hall, Pal. N. Y., I, 1847, p. 145, pi. xxxiii, fig. 12. 



This shell is only an elongate variety of R. inmquivalvis and appears 

 to be a rare form in the, Trenton of New York, yet at Curdsville, Ken- 

 tucky, it is abundant. As in R. inmquivalvis, the lamellae are well 

 developed. 



