Cyclostrema, Adeorbis, Vitrinella, and related genera. 139 



some of the Avhorls with rounded walls. Aperture oblique, nearly- 

 circular. Peritreme simple, continuous, slightly attached to the 

 body- whorl, reflected over the umbilicus. 



Operculum (fig. 8) of the type species is thin, round- 

 ovate, delicate horn-color, of few abruptly enlarging 

 whorls indistinctly defined by a spiral thread and 

 showing sinuous transverse lines of growth, nucleus 

 slightly excentric. 



The animal has a broad emarginate head with one 

 pair of long slender tentacles ; with a rather broad, 

 short, tapered, ciliated verge just beneath the base of 

 the right one. Eyes none. Gill attached to the left side lying across 

 the top of the body just within the mantle edge. Jaw plates thin, 

 delicate horn-color with a broad band of very dark brown along the 

 strongly serrate, cutting edges. Inner surface strongly reticulated, 

 as in species of Velutina. The form of these plates is quite irregular : 

 the cutting edge is oblique, forming an angle of about 135° with the 

 inner or middle, straight edge ; the distal outline is very strongly 

 sinuously curved, forming a wide, shallow upper portion and a much 

 narrower basal portion. 



The radula (pi. xxiii, fig. 16) consists of numerous rows of delicate 

 colored, rather stout, non-serrate teeth, each row having a series of 

 thirteen : — a very small central or median tooth with rather long, 

 strongly curved tip, placed a little above and alternating somewhat 

 with the rest of the series; on either side, one broad strongly hooked 

 lateral, and a much broader second lateral one with correspondingly 

 broad, more pointed hook ; be3'^ond, three, about equal, much nar- 

 rower, somewhat sickle-shaped, marginal ones with a small triangular, 

 scarcely perceptible, platelike one on the outer edge. 



The form of the shell and operculum strongly resemble a Choristes 

 elegans var. tenera Verrill of medium size, but the radula shows 

 marked and interesting differences. In that species, or rather 

 variety, there are but eleven teeth in each series, the second or outer 

 lateral tooth having a double or bilobed tip (these Trans., v, p. 541, 

 pi. lvi,figs. 27, 27a, 1882 ; vi, p. 256, pi. xxix, figs. 9-9^*, 1884. 



Type, Choristella leptalea sp. nov. 

 Figure 9. Plate XXIII. figs. 16, a. 

 Shell without sculpture, dull opaque white, of very delicate tex- 

 tui-e, consisting of about three and a half whorls. Operculum, 

 animal, and odontophore described under the genus. 

 Greatest width, about 3-5"""; height, about 2-5' 



;mm 



