50 COELOCENTRUM, SUBG. ELASMOCENTRUM. 



C. dathratum v. MART., Biologia, Moll., p. 269, pi. 16, f. 29, 

 a-c, 30 (Nov., 1897). 



Subgenus ELASMOCENTRUM Pilsbry, 1902. 



Like typical Coclocentrum in the general form of the shell 

 and the large, hollow internal column ; but the latter is smooth 

 externally, and in the last two whorls there runs a spiral 

 lamella like that developed in Anisospira. 



C. EXLEX n. sp. PI. 17, figs. 20, 21, 22, 23. 



Shell deeply rimate and perforate, cylindric-fusiform, the 

 upper third tapering, last whorl distinctly contracted under 

 the penultimate, fourth whorl from the last widest; rather 

 thin, whitish flesh-tinted. Surface lustreless, sculptured with 

 irregular, strongly arcuate riblets, about 6 to 7 in the space of 

 a mm. on the front of the penultimate whorl, about 4 on the 

 front of the last whorl. Whorls remaining 32^4, very convex 

 and very narrowly coiled, the last rounded basally and above, 

 girt by a low cord bounding the base; becoming free in front. 

 Aperture oblique, almost circular, brownish inside; the peri- 

 stome expanded, narrowly subreflexed. Length 23, diam. 7.6 

 length of aperture 4, width 4.5 mm. 



Internal column large, its greatest diam. about 3 mm., white, 

 polished and without longitudinal sculpture. Within each 

 whorl it is concave, becoming convex near the base, not con- 

 tracted below the convexity. Within the last and penultimate 

 whorls it is encircled close to the base by a sharp and rather 

 strong spiral lamella, the lower end of which may be seen 

 weakly appearing in an oblique view in the mouth. 



Mexico, exact locality unknown. 



With the narrow whorls and rather coarse sculpture of typ- 

 ical C. arctospira, this species differs conspicuously in the far 

 smaller number of whorls retained. The interior of typical 

 C. arctospira is unknown, but the form referred to that species 

 by Strebel has a pillar with longitudinal sculpture, totally 

 unlike the column of this species, which stands unique in the 

 genus, and indeed closely approaches that of Anisospira. The 

 detail figure of sculpture (fig. 23) represents the front of the 

 last and penult, whorls. 



