NATURAL SCIENCES OF PHILADELPHIA. 20T 



characters, comparing the dentition of the other species with it. 

 The centrals are longer than wide ; the base of attachment has its 

 outer, lower, lateral expansion but little developed, its lower mar- 

 gin incurved, its upper margin squarely reflected ; the reflection is 

 stout, with subobsolete side cusps, but well-developed side cut- 

 ting points, and a stout, short median cusp, bearing a cutting 

 point which does not reach the lower margin of the base of at- 

 tachment. The laterals are like the centrals, but, as usual in. the 

 genus Helix, unsymmetrical by the suppression of the inner, 

 lower, lateral expansion of the base of attachment and the inner 

 side cusp with its cutting point. The transition teeth are charac- 

 terized by the gradual lesser proportional development of the 

 reflection, and greater development of the inner cutting point; as 

 the teeth pass outward, this point becomes bifid, the reflection be- 

 comes gradually shorter, until the true marginals are reached. 

 These last are low, wide, the reflection equalling the base of 

 attachment, the inner cutting point being greatly developed, long, 

 oblique, bluntly bifid, the inner bifurcation the shorter of the two; 

 the outer cusp is very short, blunt, sometimes also bifid. In fig. 7, 

 the 10th is the first lateral showing decided modification ; the 

 14th tooth has its inner point bifid; the 17th tooth is a decided 

 marginal. The transition from laterals to marginals is so gradual 

 that it is often difficult to give the number of perfect laterals. In 

 many cases, therefore, the number given by me must be consid- 

 ered as only approximately correct. There is great variation in 

 the denticulation of the marginal teeth. 



The general character of the dentition of the other species is 

 about the same as in appressa. I found great difficulty in de- 

 tecting the side cutting points in several species, especially tri- 

 dentata and palliata. In some species I did not find the transition 

 teeth or inner marginals with bifid cutting point (pi. X., fig. 3, 4). 



I give below the count of the teeth in the several species. 



H. palliata (pi. X., fig. 2) has 34134 teeth ; 12 perfect late- 

 rals; another specimen had 14 laterals. Morse counted 115 rows 

 of teeth. The inner cutting point of the transition teeth in this 

 species is very large, as shown in c. 



H. obstrictaipLXYIU., fig. 10) has 33133 teeth; 10 perfect 

 laterals : very like H. palliata. My figures are drawn from that part 

 of the lingual membrane which has the cutting points of its teeth 



