346 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



to the front; area triangular, but wider than high, rather distinctly arched; foramen 

 very narrow, and apparently entirely open. Dorsal valve convex, but much more 

 depressed than the other; mesial fold obscure in the umbonal region, slightly elevated, 

 and rounded at the front. Surface of each valve ornamented by about forty small 

 depressed radiating costa?, some six or seven of which occupy the mesial sinus of the 

 ventral valve, and seven or eight the fold of the dorsal valve. 



Length, 0.52 inch; breadth (along hinge-line), about 0.60 inch; convexity, 0.42 

 inch. 



The costae are all simple, unless a few of them bifurcate in the mesial sinus or on 

 the fold. They generally converge to the beaks, though a portion of those near the 

 lateral extremities seem to run out on the hinge before reaching the beaks. None of 

 the specimens are in a condition to have preserved finer surface-markings, if there 



This shell is of the same type as several species found in rocks of the age of the 

 New York Hamilton group in that and some of the Western States, but seems to be 

 distinct from them all. 



* Locality and position.— .Same as last. 



Spieifer Engelmanni, Meek. 



Plate 1, fig. 1, a, b, c. 

 Spirifera Engelmanni (July, I860), Proceed. Acad. Nat. Sci. Pbilad., XII, 308. 



Shell rather small, semicircular, about twice as wide as long; hinge equaling the 

 greatest breadth, angular at the extremities. Dorsal valve depressed convex; mesial 

 fold rather narrow, but slightly elevated, flattened along the middle, and apparently 

 without plications. Ventral valve very convex in the umbonal region, sloping abruptly 

 to the sides and front; beak pointed, more or less arched; area high, triangular, the 

 hinge side being longer than the lateral slopes, which are usually somewhat angular, 

 generally rather strongly arcuate, or inclined a little backward over the hinge; fora- 

 men very narrow, apparently open to the point of the beak; mesial sinus narrow, 

 shallow, extending to the beak, flattened in the middle, and without plications. Sur- 

 face ornamented by from seven to nine depressed, rounded, simple plications on each 

 side of the fold and sinus. 



Length of hinge, about 0.66 inch; diameter from hinge to front, 0.39 inch; height 

 of area, 0.26 inch. 



It is probable the surface was also marked with very fine striae, and possibly 

 granules, as is not uncommon in this section of the genus, but the specimens are not 

 sufficiently well preserved to have retained such delicate ornaments, if they existed. 



This species is quite similar in size and form to the last, but may be readily dis- 

 tinguished by its much larger and less numerous plications, none of which are defined 

 on the mesial fold, or in the sinus, as in that species. As near as can be determined 

 from a description without figures or measurements, it seems to be also related to S. 

 fornacula of Hall, from the Hamilton group in Illinois (Report Regents University of 

 New York, 1857, p. 115), but has not more than half as many plications. 



