WEST AMEKICAN PYEAMTDELLID MOLLUSKS. 131 



axial ribs on the posterior half of the whorls. Of these there appear 

 to be about 18 upon the third turn. The whorls are marked by 

 spiral lines of well-impressed pits, of which 6 appear upon the first, 

 7 upon the second, 8 upon the third and fourth, 10 upon the fifth, 

 11 upon the sixth, 14 upon the seventh, and 18 upon the penultimate 

 turn, between the sutures. Upon the last, they are more or less 

 irregular. Sutures poorly marked. Periphery of the last whorl 

 well rounded. Base moderately long, well rounded, marked by 11 

 spiral lines of pits. Aperture oval; posterior angle acute; outer 

 lip thin, showing the external markings within; columella moderately 

 strong, very strongly curved, reenforced by the base. 



The type and another specimen (Cat. no. 162699, U.S.N.M.) comes 

 from San Hipolito Point, Lower California. The type has nine post- 

 nuclear whorls and measures: Length 5.5 mm., diameter 1.2 mm. 



Genus ODOSTOMIA Fleming. 



Odostomia Fleming, Edinburgh Encycl., vol. 7, 1817, pt. 1, p. 76.— Odontostomia 

 Jeffreys, Mai. and Conch. Mag., 1839, p. SS.=^Turritostomia Sacco, Moll, del 

 Piemonte e della Liguria, 1892, p. 41, same type. 



Shell with sinistral apex, usually short, few whorled, subconic or 

 ovate, with a single columellar fold which varies in strength and 

 sometimes is not apparent at the aperture. The sculpture varies 

 from smooth to lamellar axial ribs and spiral keels. 



Type. — Turbo plicatus Montagu. 



Of the forty subgenera now recognized under Odostomia nineteen 

 have been found represented on the west coast of America. 



KEY TO THE SUBGENERA OF ODOSTOMIA. 



Early post-nuclear whorls sculptured differently from the later ones. Lysacme, p. 132. 

 Post -nuclear whorls sculptured similarly throughout. 



Varices present Salassiella, p. 133. 



Varices absent. 



Axial ribs present. 



Axial ribs rounded. 



Spiral markings absent or if present consisting of microscopic stria- 



tions only Salassia, p. 134. 



Spiral markings consisting of several to many raised threads. 



Spiral sculpture less strong than axial, not nodulous. £fsZa, p. 135. 

 Spiral sculpture equal to the axial, nodulose between the su- 

 tures Chrysallida, p. 136. 



Spiral markings consisting of incised lines. 



Spiral lines on spire and base Pyrgulina, p. 169. 



Spiral lines on base only Egila, p. 170. 



Axial ribs lamellar. 



Sphal sculpture lamellar. 



Intersection of axial and spiral sculpture c-uspidate, 



Ilaldra, p. 171. 

 Intersection of axial and spiral sculpture not cuspidate, 



Ividella, p. 172. 



