324 CHORISTIDvE-HOMALOGYRIDJE. 



tacular processes ; a papilla on either side between the mantle and 

 foot, aud two posterior threads below the operculum. Jaws elliptic. 

 Radula with three rachial teeth, of which the middle is very small, 

 the others unicuspid, lateral tooth large, bicuspid, marginal small, 

 curved, simple, a small basal plate exterior to them (PI. 54, fig. 5). 



Genus CHORISTES, Carpenter. 1872. 



The characters are those of the family, the systematic position of 

 which is very uncertain. Verrill is inclined to place it among the 

 tectibranchs, near Actseon, but the shell and appendages of the 

 epipodium recall Rissoid characteristics. 



C. ELEGANS, Carp., described from Canadian post-pliocene fossils, 

 has been found living on the New England coast, within the egg 

 capsule of a ray. 



FAMILY HOMALOGYRID^. 



Body flattened, ciliated in front, no tentacles, eyes sessile (PL 

 54, fig. 3). Radula narrow, the central tooth having a quadrangu- 

 lar base with large cusp, imbricated, one series on the other, a 

 lateral, transverse plate replaces the lateral and marginal teeth. 

 (PI. 54, fig. 4.) 



Shell planorbiform, spire involute, mouth clasping both sides of 

 the periphery. Operculum corneous, few-whorled, nucleus central. 



Genus HOMALOGYRA, Jeffreys. 1867. 



Characters those of the family. H. NITIDISSIMA, Forbes & 

 Hanley. Europe, Greenland. 



The external characters of the animal and its dentition are pe- 

 culiar, and its systematic position is uncertain. 



Omalogyra, Jeffreys, and Ammonicerina, Costa, are synonyms. 



