NO. 3 BELLEROPHONT GASTROPODS KNIGHT 5 



Family BELLEROPHONTIDAE McCoy, 185 1 

 Genus CHALAROSTREPSIS Knight, new genus 



Genotype, Chalarostrepsis praecursor Knight, new species. 



Diagnosis. — Discuslike bellerophontif orm gastropods with a shallow, 

 narrow sinus in the anterior lip culminating in a deep, narrow slit that 

 generates a selenizone. The final whorl is out of contact with the coil. 

 The shell is entirely symmetrical. 



I am placing Chalarostrepsis in the Bellerophontidae close to 

 Tropidodiscus { — Oxy discus Koken) which may well be derived 

 directly from it. Tropidodiscus differs principally in that it is closely 

 coiled throughout or even moderately involute. 



CHALAROSTREPSIS PRAECURSOR Knight, new species 

 Plate i, figures 2a-c 



Description. — A moderately large species of 3 or 4 whorls with 

 sharply rounded but not angular dorsum, with the whorls of the 

 coil barely in contact, and the last f whorl free. The umbilici are, 

 of course, widely open. The slit is almost | whorl deep, narrow, and 

 generates a convex selenizone bordered by faint striae. The aperture 

 projects slightly behind in a skirtlike trail, then passes forward on 

 the sides at an angle about 40° above the radius until it approaches to 

 about one-sixth of the distance to the dorsum where it turns roundly to 

 form the margins of the slit. The ornamentation consists of growth 

 lines alone. 



Measurements 



Diameter Thickness 



The holotype (U.S.N.M. 114282) ii.o mm. 4.3 mm. 



A small paratype (U.S.N.M. 114283) 5.5 mm. ? 



A large imperfect paratype (U.S.N.M. 114283) 20 mm.* 9 mm.* 



* With estimates to allow for imperfections, especially in thickness. 



Hypodigm. — Three specimens. 



Remarks. — The only three specimens of this species known are 

 preserved in a gray, crystalline limestone. The holotype has been 

 largely removed from the matrix and is exquisitely preserved. It 

 shows shape and surface features with remarkable fidelity. The 

 slit can be seen as that adapertural portion of the dorsal crest through 

 which the matrix within the shell was continuous before preparation, 

 a roughened portion without a selenizone. The largest specimen is 

 too poorly preserved for effective preparation and the smallest is too 

 small to justify the labor. On the largest specimen the rear end of 



