144 GEOLOGICAL SURVEY OF THE TERRITORIES. 
the upper portion of the sides so that upon the space just below each 
ear they have a distinetly backward curv mG Right valve unknown. 
Height from base to beak, 30 millimeters; breadth, 26 millimeters. 
Position and locality. The type of this’ species was discovered in 
Jurassic strata at the north base of Aquarius Plateau, Southern Utah. 
Professor Whitfield also reports its existence (loc. cit.) in Jurassic strata 
of the Black Hills, but refers it to the C. extenuatus of Meek & Hayden 
The last-named species, however, as deseribed and figured by its authors 
is a Smooth or nearly smooth one, and has a different marginal outline 
Genus AVICULOPECTEN McCoy. 
AVICULOPECTEN? SUPERSTRICTUS (sp. nov.). 
Plate 37, figs. 4 a and b. 
Shell suborbicular in marginal outline; cardinal border rather long, 
straight, and at nearly right : angles with the axis of the shell; the free 
margins below the ears forming the greater part of a nearly perfect 
circle. Left valve moderately eibbous, the convexity being nearly uni- 
form below but narrowing laterally toward the umbo, which is somewhat 
prominent and arched ; beak small and projecting only slightly beyond 
the hinge margin ; ears moderately large and well defined, but not sep- 
arated trom the body of the shell by auricular furrows; posterior ear 
having its outer margin slightly concave and its outer angle somewhat 
acute; anterior ear a 1 little - more abruptly flattened than the other, its 
margin forming a concavity with that of the body of the shell and being 
a little convex at the outer extremity. The body of the valve bears 
from twelve to fifteen slender radiating ribs between which each space 
is occupied by from five to seven radiating, thread-like, raised lines, which 
increase in number by implantation with the growth of the shell, and 
the first-formed lines also increase slightly in size. Indeed, the ribs 
themselves in their origin consist of just such lines as are formed be- 
tween them in the after growth of the shell. Both ears are marked by 
lines similar to those between the ribs. Right valve and hinge unknown. 
Height from base to cardinal border, 29 millimeters; transverse diam- 
eter, 50 millimeters; length of hinge line, about 26 millimeters. 
This sheli is probably not a true Aviculopecten, but in external char- 
acteristics it closely resembles certain forms of that genus; and as noth- 
ing is yet known of its hinge, it is referred provisionally to Aviculopecten 
in consequence of those external characteristics. It is doubtless con- 
generic with the three Triassic species which, in Contributions to Inverte- 
brate Paleontology No. 5, on previous pages of this volume, I have 
referred pr ovisionally to that genus. It agrees with those three species 
in the character of its surface markings, but its hinge margin is not 
perceptibly oblique with the axis of the shell. This latter character dis- 
tinguishes it from A. idahoensis and A. pealet. A. altus is very slightly, 
if any, oblique, but that species differs from the form here described in 
being proportionally much higher from base to beak, and also in having 
its left valve less convex and its hinge line proportionally shorter. The 
shell here described comes from the : same region in which the Triassic 
fossils just referred to were discovered, and it is possible that it comes 
also from Triassic strata; but from information received I at present 
refer it to the Jurassic. 
Position and locality—Head of Lincoln Valley; from strata supposed 
» pe of Jurassic age, where it was discovered by Prof. O. St. John, in 
7 
