210 PROCEEDINGS OF THE ACADEMY OF 



go, I cannot find it. The outer laterals, however, in most of the 

 species have a much more developed side cusp than the inner 

 laterals, bearing a well-developed cutting point (see pi. XI., fig. 

 6, d): but not all the species, as H. exoleta for instance, as shown 

 in pi. XI., fig. 1, d, has no well developed side cusp and cutting 

 point on its outer laterals, nor does it appear except on the de- 

 cided marginals. It is the same in H. Sayii. 



I find also variation in the manner of passing from the lateral 

 to the marginal teeth among the species of this first group of Me- 

 sodon. In H. exoleta (pi. XL, fig. 7, 14) the cutting point remains 

 the same, and also in Sayii, profunda, Wetherbyi, and Mitchel- 

 liana, but in elevata (pi. XII., fig. 1, 18) the transition teeth are 

 characterized by the bifurcation of the large cutting point; the 

 same occurs in albolabris, viultilineata, Roemeri, Columbiana (pi. 

 XII., fig. 2) and dema, and the rest of the gi'oup. 



The general character of the teeth in this section of Mesodon is 

 about the same as I have described above for Triodopsis (p. 260). 

 It will be noticed, however, that the marginals (as in H. exoleta 

 and Wetherbyi) do not always have their cutting points bifid. 



The other type of dentition in the subgenus Mesodon is shared 

 by H. thyroides (pi. XVIII., fig. 2), claiisa (pi. XII., fig. 7), and 

 Wheatleyi (pi. XVIII., fig. 1). The centrals and first laterals have 

 subobsolete side cusps without cutting points, the outer laterals 

 have no side cusp, but retain the type of the first laterals, they are 

 much longer, narrower, and have one extremely long, oblique, 

 stout, bluntl}' pointed cutting point, reaching far beyond the 

 lower margin of the base of attachment. These outer laterals 

 pass gradually into the marginals, which retain their general form 

 but have a less developed reflection, and much more proportion- 

 ally developed cutting point, sometimes bifid in the extreme mar- 

 ginals (pi. XVIII., fig. 2, 54), and usually with a small side cutting 

 point. 



As in all the subgenera of Helix, the marginal teeth of Mesodon 

 show great variation in their denticulation, even in most cases on 

 the same membrane. H. Glarki (pi. XL, fig. 6) has the marginals 

 with cutting points much blunter and broader in some parts than 

 in the others. 



The study of the dentition of Mesodon shows that we must be 

 prepared to find considerable variation in the character of the 

 teeth of any subgenus. The peculiar outer lateral teeth and mar- 



