NO. 2094. M0LLU8K8 OF THE GENUS RISSOINA—BARTSCH. 43 



RISSOINA FAVILLA, new species. 

 Plate 28, fig. 3. 



Shell of medium size, bluish white, semitranslucent. Nuclear 

 whorls decollated, except the last turn, which is smooth. Post- 

 nuclear whorls flattened in the middle, well rounded at the shoul- 

 dered summit, marked by strong, very regular, somewhat curved 

 pro tractive axial ribs, of which 14 occur upon the first and second, 18 

 upon the third, 20 upon the fourth, 22 upon the fifth, 26 upon the 

 sixth and penultimate turn. These ribs extend prominently to the 

 summit and render the sutures feebly wavy. Intercostal spaces as 

 wide as the ribs, crossed between the sutures by very many, exceedingly 

 fine, spiral striations. Sutiures strongly marked. Base of the last 

 whorl weU rounded, marked by the continuations of the axial ribs 

 which extend prominently to the umbihcal chink and by about ten 

 rather distantly spaced, spiral threads between which numerous, very 

 fine, spiral striations occur. These threads extend prominently across 

 the intercostal spaces, becoming obsolete on the ribs. Aperture 

 rather large, very obhque, channeled anteriorly and posteriorly; outer 

 hp thick, reinforced immediately behind the edge by a strong varix; 

 inner Hp appressed to the base, thick, sinuous, so arranged as to give 

 the aspect of having a partially truncated columella. Peritreme 

 complete. 



The type. Cat. No. 4162&, U'.S.N.M., comes from Cape San Lucas. 

 It has the last nuclear whorl and 8 postnuclear turns and measures: 

 Length, 7.1 mm.; diameter, 3 mm. Cat. No. 4062a, U.S.N.M., con- 

 tains 2 specimens from the same locahty. Cat. No. 46160, U.S.N.M., 

 1 specimen from Santa Maria Bay, Lower Cahfomia. 



RISSOINA MAZATLANICA, new species. 

 Plate 30, figs. 1, 3. 



Shell small, elongate-conic, milk white. Nuclear whorls small, If, 

 formmg a depressed spire which gives the apex of the shell a truncated 

 appearance. Post-nuclear whorls moderately roimded, appressed at 

 the summit, marked by moderately strong, quite regularly disposed, 

 rounded axial ribs, of which 14 occur upon the first and second, 16 

 upon the third, 18 upon the fourth, 24 upon the fifth, and 26 upon 

 the penultimate tiu-n. These ribs become somewhat enfeebled 

 toward the summit, where they render the suture feebly sinuous. 

 Intercostal spaces about as wide as the ribs, crossed between the 

 sutures by numerous microscopic striations. Sutures feebly im- 

 pressed. Base rather short, well rounded, marked by the continua- 

 tions of the axial ribs and 13 equal, and equally spaced, slender, spiral 

 threads which extend prominently across the intercostal spaces and 



