ODOSTOMIA. 265 



The body is usually to the spire as two to five (in the move 

 produced form as one to three); its basal declination is 

 more or less abrupt, somewhat flattened when the peri- 

 phery is angulated, somewhat rounded when this is not the 

 case. The mouth occupies from one-third to two-fifths of 

 the entire length ; the general character of it is to be short 

 in proportion to its breadth ; it is of a subrhomboidal ovate 

 figure, especially in the more typical examples, wherein, 

 besides that the pillar lip, which is always straight and 

 never much elongated, meets the straightish base of the 

 penult turn at an obtuse angle, and unites at almost a 

 rectangle with the outer lip, this last, which is acute, 

 simple, and more or less projecting below, being straightish 

 above, forms likewise a rounded off angle with the basal 

 line. The throat is perfectly smooth. The sharply pro- 

 jecting tooth-like fold is horizontally compressed, and lies 

 almost in the middle of the inner lip. There is no um- 

 bilicus, but at most a chink. The columella is perpendicu- 

 lar, and the pillar lip curls over it (particularly in front) 

 but is not quite appressed, the extreme edge being clearly 

 defined. It is very seldom that individuals attain to the 

 length (the fifth of an inch) ascribed to them by Montagu, 

 who, we suspect, included conoidea with this species ; 

 the majority of our own examples are not above a line 

 and two-thirds long, with a basal diameter of less than one 

 half of this measurement. 



The 0. unldentata has the general aspect of Rissoa ulva:^ 

 and approaches closely to the O. conoidea and acuta. The 

 smaller size, the less planulate volutions, the more bluish 

 tint of whiteness, and, above all, the smoothness of its 

 throat, distinguish it from the former; the absence of 

 both colour and umbilicus, its less acutely slender shape, 

 and the greater straightness and reflection of its pillar lip, 



VOL. iir. M M 



