23 



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

 ordinary lines of growth. 



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

 volution, twenty -thee millimeters. 



This shell differs from other species 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 Gamp Cottonwood, 

 Spring Mountain range, Nevada. 



Genus DENTALIUM Linnaeus. 



DENTALIUM CANNA (sp.-nov.) 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 striaB. 



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 ftase, 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 Spring, Arizona. 



MESOZOIC. 

 JURASSIC PERIOD. 



CONCHIFERA. 

 Genus CAMPTONECTES Agassiz. 



CAMPTONECTES STYGIUS (sp. nov.) 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 oblique with the axis, which 

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

 because a trifle more than one half the width of the shell is in front of it ; 

 the margins of the lower two-thirds regularly rounded, the basal portion 

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

 tion. 



Right 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 strire of growth are 

 visible, and in a favorable' light indications of radiating costte may 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 aud 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 North Fork of Virgin River, Utah." 

 Rocks of Jurassic age. 



