27 



Diameter, longitud. seven eighths of an inch ; vertical four fifths 

 of an inch ; transverse three eighths of an inch. 



Inhabits Mazatlan. Col. Jewell. 



This species is so similar to the well known T. carnaria, of the 

 West Indies, that it is best described by a comparison with it. 

 Its form usually is more rounded ; striae less numerous and larger, 

 forming a more decided angle on the posterior slope ; so that on the 

 disk they are more nearly vertical ; and on the anterior slope the 

 angle they form is so acute as to render them confused along the 

 margin, nor have they any undulations at this border. The interior 

 is deep carmine throughout, whereas C. carnaria is more roseate, 

 and the border is always white ; cardinal teeth of left valve less 

 divaricate. Fig. 38, of Sowerby's Thesaurus, seems to represent 

 this shell. 



CYREXA ALTILIS (PI. XVI. Fig. 5.) 



T. solidiuscula, subtriangularis. subequilateralis, ventricosa, tenue striata, calca- 

 rea, epidermide tenui cinerea induta ; umbonibus antemedianis valde elevatis, de- 

 clivitate umbonali angulato ; facie dorsali antico cordiformi, excavate, margine 

 recto, ad extreraitatem citb rotundato ; margine dorsali postico arcuato ad api- 

 cem subtruncato : limbo marginal! interior! violacescente, submargine incarnato. 



Shell solid, obliquely subtriangular, ventricose, calcareous, co- 

 vered with a thin ashy-green epidermis ; beaks a little antemedian, 

 elevated, pointed, incurved and antrorse ; posterior dorsal slope 

 decidedly angular, anterior much less so ; disks gibbous ; ventral 

 margin regularly arcuate : anterior dorsal aspect cordiform, some- 

 what excavate, margin rectilinear, extremity briefly rounded ; pos- 

 terior dorsal margin somewhat convexly arcuate and compressed, 

 and more broadly rounded than the anterior ; hinge well developed ; 

 cavity of beaks profound ; hinge margin white, disk bluish white, 

 region of pallial impression flesh colored ; margin violaceous, 

 simple. 



Long. diam. one and three eighths inches ; vertical diam. one 

 and a half inches ; transverse diam. one and one fourth inches. 



One specimen is marked " Mexico," by Maj. Rich ; another, 

 " Mazatlan (?) " by Col. Jewell. 



Compared with C. solida, with which it is nearly related, it is 

 less solid, the surface comparatively smooth, and the epidermis 

 smooth, the beaks much more elevated, the umbonal ridge sharper, 



