LOGAN: FREEZE-OUT HILLS OF WYOMING. 131 



specific characters are difficult to determine. The arms are a fraction 

 over an inch in length, and are elevated along the central portion. 

 They are composed of small plates arranged longitudinally. The 

 center of the body is marked by a depression which appears to 

 be almost circular, although the margin is not entire, being more- 

 or less obliterated by weathering. In general shape the upper sur- 

 face of the rays are subangular. The rays are not so curved as those 

 in the specimen described by Whitfield*; it may be that the first is 

 an older individual. 



Geological horizon : The species occurs in a thin stratum of sand- 

 stone in the bluish shale of No. 15 of the section. It is found asso- 

 ciated with Pentacrinns astericKS, Belemnites densvs. etc. The 

 sandstone in which it occurs is fine-grained, laminated stone, which i.« 

 free from impurities and varies much in degree of induration. 



Locality : Freeze-out Hills, "Wyoming : also reported from the- 

 Black Hills. 



Remarks : The above species is not abundant in the Jurassic strata 

 of the interior, as so far it has been reported only from the Jura of 

 Wyoming. These forms, however, are not easily jDre-served, and the 

 depositional conditions of the shallow Juras.sic sea were never very 

 favorable to their preservation. 



^ivicitln moci'OHotHs Meek. Plate XXX, fig. 6. 



Shell medium in size, arcuate, ornamented with two wings, the pos- 

 terior one of which is much larger than the anterior one. The pos- 

 terior wing bears a broad, shallow sinus, while the anterior wing is 

 small and somewhat rounded. The hinge line is mod>'rately long. 

 The surface of the shell is ornamented with strong radiating plica- 

 tions or ribs, which are crossed by well-marked lines of growth. 

 These form, at the points of intersection, nodes which are prominent 

 on the lower part of the shell but scarcely visible in the umbonal re- 

 li'ion. The umbonal region is the area of greatest convexity. The 

 anterior margin of the shell projects slightly in the upper part so as 

 to leave a faint sinus below the anterior wing. It curves obliquely 

 l)ackward to the basal border below, with which it forms an almost 

 regular curve extending nearly to the posterior basal extremity. The 

 last-named i3ortion of the shell is also rounded. The beak is sharply 

 pointed, shows a tendency to incurve, and is elevated above the car- 

 dinal line. 



Dimensions: Length, 20 mm.; width, 12 mm.; height, 5 mm. 



Greological horizon : This species occurs in tlie thin sandstone 

 stratum of No. 15 of the section. It is found associated with Asterias 

 'liihium, Pentacrinvs asterici'S, and Belemnites densi'S. 



* Geology of the Black Hi Is. 

 lO— K.U.Qr. A-ix 2 



