394 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.42. 



rounded at the base. The beak, however, differs from Sowerby's 

 species in its more central position and greater extension over the 

 hinge area. The anterior and posterior slopes of the surface are 

 nearly the same; a broad shallow sulcus separates the ear from the 

 body of the shell. The right valve is convex; beak strong, narrow, 

 and overarching ; umbonal ridge at first narrow and subcarinate, but 

 broadening out and flattening toward the base. The anterior slope 

 is sUghtly concave between the umbonal ridge and the anterior ear. 

 In one of the smaller specimens, a mold of the interior of a right 

 valve, there is a rather sharp boundary to this sulcus, without, how- 

 ever, forming distinct ridges as in the case of Cliopteria hicostata. 

 The dentition of hinge is as in Pterinea, with a long, somewhat curved 

 linear posterior lateral tooth, and a few closely approximated oblique 

 denticles under and a little anterior to the beak. 



Locality. — The type-specimens are from the Silurian shales at the 

 head of Leighton Cove, Pembroke, Washington County, Maine. 



Cotypes.—C&t. No. 58443, U.S.N.M. 



Genus STREPTOTROCHUS Perner. 



Jaroslav Perner described the genus Streptotrochus in the year 1907,^ 

 as follows: 



Coquille etroitement ombiliqu^e, k test mince et a tours l^gerement aplatis, qu^ 

 ne se touchent sou vent un peu qua la base. Les tours possedent une section trans- 

 verse subtriangulaire et tme peripheric anguleuse. Stries transverse ^cailleuses, 

 faiblement courb^es; plis en spirale, peu marques. 



Type: Streptotr. rugulosus Barr, sp. (pi. 106, figs. 11-15) (Gist, et local. Bande E2, 

 Gross-kucbel, Locbkov.). 



The author recognized the following species as congeneric: Strepto- 

 trochus mercurius (Barrande),^ Trochus incisus Lindstrom,^ and with 

 doubt Troclms lundgreni Lindstrom.^ 



To the genus Streptotrochus I refer some forms abundant in some 

 of the shales on Moose Island associated with abundant representa- 

 tives of the genus Eurymyella. Our species are smaller than the 

 type-species from Bohemia and agree most nearly with Lindstrom's 

 species TrocTius incisus, but differ from that species in the less rapidly 

 expanding spire. The form of the whorls is extremely variable, so 

 that it is almost impossible to pick out any one specimen as a specific 

 type to stand for other specimens crowded closely together with it 

 on a single slab. In some of the more regular forms the resemblance 

 to Lindstrom's figures of Trochus incisus (numbered 24 and 25 on 

 his pi. 14) is very close. Our shells are slightly narrower and 

 smaller in size. 



1 Syst. Sil. du Center Boheme, pt. 1, vol. 4, Gasteropodes, vol. 2, 1907, p. 238. 



2 Idem, p. 239, pi. 106, figs. 20-21. 



3 Kongl. Svensk. vet.-acad. Handl., vol. 19, 1884, p. 151, pi. 14, figs. 22-31. 

 < Idem, p. 149, pi. 14, figs. 46-63. 



