25() PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



several strong-, rounded ribs irregularly alternating with smaller ribs; 

 fine, radiating strife occur on the ribs and interspaces. 



Only two specimens of somewhat imperfect ventral valves are known 

 of this species. The description is of the best preserved specimen. 

 The second specimen has a greater lumiber of more regular rilis that 

 increase by interpolation of short rilxs. One specimen has a width of 

 8 nmi. and a length of -1.5 mm. from the apex to the front margin, 

 which is about the distance from hinge line to front margin. 



The nearest related species is Wlsusla alherta^ from which it differs 

 in surface ribs, forward inclination of the area, and prominent convex 

 deltidium. 



Formation and locality. — Middle Cambrian. Limestone about 50 

 feet above top of upper greenish shales, Rock Canyon, back of Provo, 

 Utah. 



NISUSIA (JAMESELLA), species undetermined 



Thisdoul)tful species is l)ased on an interior cast of a liroken ventral 

 valve. The width of the shell is 15 mm., and length 8 mm., with apex 

 broken off'. A broad, shallow sinus arches the frontal margin, and 

 narrows toward the umbo. Radiating ribs narrow, about 5 in a space 

 of 2 mm. 



Formation and locality. — Lower Cambrian. Pinkish-colored lime- 

 stone of Olenellus zone, coast of Labrador on Straits of Belle Isle. 



EOSTROPHOMENA, new subgenus of STROPHOMENA. 



The description of the type species gives all that is known of the 

 subgenus. It differs from Strophomena in having the cardinal process 

 in the dorsal valve merged into a ridge formed by the extension of the 

 crural plates along the posterior margin, so as to cover the delthyrium 

 and unite at the median line. 



Type. — Strojdioinena {Font rophomen a) elegantala.^ new species. 



Ohscrvations. — The cardinal process is more like that of Orthothetes 

 than that of Strophomena; it differs from both in being more simple 

 in its construction. It is probable that there is a group of shells hav- 

 ing the characters of *S'. [E.) elcyantula., but owing to the difficulty of 

 obtaining interiors of the valves, it is impossible to designate them. 

 Strojdwniena auro7'a Billings" is one of these; also Leptxna decipiens.^ 



STROPHOMENA (EOSTROPHOMENA) ELEGANTULA, new species. 



(xeneral form transversely sul)semicircular or subelliptical, greatest 

 width at the straight hinge line. Surface concavo-convex, and marked 

 t)V tin(% radiating, even stria% separated into flat bands b}^ stronger 

 and mor(^ prominent stria> oi- ribs; fine concentric strife, and more or 

 less prominent lines of growth cross the radiating strire. The ventral 



"Pal. Foss., I, p. 218. ''Idem., p. 74. 



