23 



line with the axis of the shell. Surface apparently smooth, except the 

 ordinary lines of growth. 



Length of the largest specimen, thirty-six millimeters ; width of last 

 volution, twenty-three millimeters. 



This shell ditters from other si)ecies of Macrocheilus in the possession 

 of such an angle as has been described at the upper part of the whorls. 



Position and locality. — Carboniferous strata at Camp Cottonwood, 

 Spring Mountain range, Xevada. 



Genus Dent.vliu3I Linnieus. 



Dentalioi canna (sj). nor.)— Shell large, thin, straight or slightly 

 curved ; section circular or nearly so ; surface marked by numerous 

 encircling lines of growth crossed by fine, obscure, longitudinal striae. 



Our specimens indicate that individuals of this species reached a 

 length of ten or twelve centimeters. The largest fragment measures 

 nine millimeters in diameter at the base, and at a distance of five centi- 

 meters toward the apex, the diameter is six millimeters. 



The great size of this species, together with its delicately-marked sur- 

 face, clearly distinguishes it. 



Position and locality. — Strata of the Carboniferous period, near Salt 

 Lake, New ^Mexico, and near Relief Si)ring, Arizona. 



ME80Z0IC. 



JURASSIC PERIOD. 

 COXCHIFEKA. 



Genus Camptonectes Agassiz. 



Camptonectes stygius {sp. nor.) — Shell of medium size, thin, lentic- 

 ular; length of the hinge-line a little more than half the height of the shell 

 from basal margin to beak, very slightly obIi(]ue with the axis, which 

 inclines a little posteriorly, althougii it does not at first view appear to, 

 because a trifie more than one-half the width of the sln^ll is in front of it ; 

 the margins of the h)wer two-thirds regularly rounded, the basal portion 

 having rather a shorter curve than either tlie anterior or posterior por- 

 tion. 



llight valve smooth, depressed-convex ; posterior ear rather small, 

 plain, its outer border forming a somewhat obtuse angle with the cardinal 

 border ; anterior ear moderately large, prominent, proportionally nar- 

 row, the upper and lower sides approaching each other at an acute angle ; 

 anterior border extending farther forward than the extremity of the 

 anterior ear, at which part it is abruptly rounded and then continued 

 backward and upward almost straight to the bottom of the byssal notch. 



Surface nearly smooth, but concentric lines and striii? of growth are 

 visible, and in a favorable light indications of radiating costie maj- also 

 be observed. 



Height from base to beak, forty-one millimeters ; width, forty millime- 

 ters; length of hinge-border, twenty-three millimeters. 



This shell resembles C. bellistriatus Meek and Hayden, from the Juras- 

 sic strata of Dakota, but it differs in the outline of the borders, the shape 

 of the ears and byssal notch, and in the surface-markings. 



Position and locality. — " Edge of the bluff, fifteen to twenty miles south 

 of Dirty Devil River, and upon the Xorth Fork of Virgin River, Utah." 

 Rocks of Jurassic age. 



