NO. 1908. NEW SILURIAN MOLLUSCA FROM MAINE— WILLIAMS. 393 



ridge followed on its anterior side by a strong ridge on the anterior 

 slope. Excei>t for these ridges and furrows, the surface is apj)arently 

 smooth. 



Locality. — Limestone band in the shales at northwest corner of 

 Youngs Cove, Pembroke Township. 



Eolotijpe.—C&t. No. 58441, U.S.N.M. 



CLIOPTERIA UNICOSTA, new species. 



Plate 50, figs. 3, 4, G. 



Right valve shorter and more gibbous than C. hicostata, and the 

 cardinal area higher and broader. The umbonal slope arched and 

 forming the highest part of the shell, in front of which and separated 

 from it by a narrow furrow is a single cord-hke ridge proceeding 

 from tip of beak to the ventral margin, though this ridge less 

 prominent and does not rise as high as the umbonal ridge; whereas 

 in C. hicostata the ridge forms the highest elevation of the shell. 



On tlie anterior slope there is a broad, shallow sulcus, the surface 

 rising gently before reaching the ear, but not forming a distinct second 

 cord-Uke ridge as in the other species. In the specimen figured (pi. 50, 

 fig. 6) there is a narrow, triangular area separating the broad, flat- 

 tened area under the beak from the curving posterior slope of the 

 surface. This narrow triangular area is depressed and bounded on 

 both sides b}^ slightly raised lines. In another specimen this trian- 

 gular area is curved and separates the flat cardinal area from the 

 curving side of the shell and consists of two strong bounding thread- 

 like ridges, between which are two finer parallel lines. The left valve 

 is gibbous, but not as highly so as the right, and is marked on the 

 anterior slope by a single low ridge separated by a faint narrow 

 furrow from the umbonal ridge, which is broad and evenly rounded 

 and separated from the small anterior ear by a broad, shallow sulcus. 



The surface is smooth except for occasional concentric growth 

 lines. In one mold of the exterior are seen also faint radiating 

 lines. The two species are found in the same limestone and are 

 clearly congeneric, the chief difference consisting in the long narrow 

 form and more pronounced ridges of the one (C. hicostata) and the 

 short gibbous form \\dth fainter ridges of the other {C. unicosta). 



Locality. — Limestone bands in the shales at northwest end of 

 Young's Cove, Pembroke. 



Cotypes.—Qdii. No. 58442, U.S.N.M. 



PTERINEA LAXATA, new species. 

 Plate 50, figs. 5 and 5a. 



Tills species resembles Avicula rectangularis Sowerby in its general 

 outline and in the form of the left valve. It is subtriangular, convex, 

 with high subcarinated umbonal ridge; surface smooth; posterior 

 mai^in nearly at right angles with the cardinal margm, broadly 



