172 PROCEEDINGS OF THE ACADEMY OF 



The reflection is short, with three distinct cusps, the median long 

 and slender, bulging at the sides, the outer ones very short; all 

 the cusps bear cutting points in proportion to their length. The 

 lateral teeth are arranged in straight transverse rows. They are 

 like the centrals, but unsym metrical by the partial suppression of 

 the inner side cusp and inner lower lateral expansion of the base 

 of attachment, and the complete suppression of the cutting point 

 to the inner side cusp. The marginals have a sole-shaped base of 

 attachment, and truly aculeate cutting points, which, however, 

 are bluntly bifid at their points. The marginals are in oblique, 

 curving rows, gradually decreasing in size of the teeth as they 

 pass off laterally. They do not first increase and then decrease, as 

 in Zonites and Glandina, or not, at all events, to the same degree. 

 In V. limpida, as stated below, the seventh marginal appears, 

 however, to be the largest. 



In V. limpida I have counted 30 1 30 teeth, with 9 perfect 

 laterals. The seventh marginal is the largest. Another gave 

 39 1 39, with 10 perfect laterals. The membrane figured by 

 Morse had 25 1 25 teeth, with 9 laterals. I have figured of this 

 species on pi. IV. one central and its adjacent lateral in fig. 8 a, 

 and the twenty-third tooth, which is one of the marginals, in 

 fig. 8 6. 



Vitrina exilis has about 37 1 37 teeth, with 7 perfect laterals. 

 I have given on pi. IV., fig. 7 a, one central and lateral ; 6, a group 

 of marginals; c, an extreme marginal. 



Vitrina Pfeifferi has over 50 1 50, with 10 perfect laterals. 

 I figure a group of centrals and laterals, pi. IV., fig. 6 a, and one 

 extreme marginal in b. 



Genus UMAX, Lin. 



The character of the mantle and the peculiarities of the lingual 

 dentition have suggested various subdivisions of this genus into 

 sections, sub-genera, and even genera. I propose, however, to 

 consider the genus in its widest sense, as generally adopted. It 

 will be seen that even in the few species existing in North Ame- 

 rica, there is considerable variation in the lingual dentition, espe- 

 cially in the bifurcation or non-bifurcation of the marginal teeth, 

 the development of the side cusps to the central and lateral teeth, 

 and the presence or absence of distinct cutting points to these 

 cusps. I shall, however, simply describe the dentition of our 



