MESODON. 3 id 



divesta, Clarki. Even among these species there are some important raria 

 Thus I have failed to detect any side cutting points on the subobsolete side cusps 

 of the central and first lateral teeiii of Roemeri, Wetherbyi, Downieana, Sayii, 

 exoletu, Pennsylvanica, and Mitchelliana. All these species have their side cusp 

 less developed than in the other species mentioned above. The presence of 

 the cutting point may be detected by better manipulation than I am able to 

 give, but as far as my powers go, I cannot find it. The large median cutting 

 point, however, has a decided lateral bulging, which is readily mistaken for a 

 distinct side cutting point, and indeed replaces it. 1 The outer laterals, how- 

 ever, in most of the species have a much more developed side cusp than the 

 inner laterals, bearing a well-developed cutting point (Fig. A, Fig. 16), but 

 not all the species, as some have no well-developed side cusp and cutting point 

 on their outer laterals, nor does it appear except on the decided marginals. It 

 is thus in M. SayiL 



I find also variation in the manner of passing from the lateral to the mar- 

 ginal teeth among the species of this first group of Mesodon. In M. exoleta the 

 cutting point remains the same, and also in Sayii, profunda, Wetherbyi, and 

 Mitchelliana, but in elevata the transition teeth are characterized by the bifur- 

 cation of the large cutting point ; the same occurs in albolabris, multilineata, 

 Roemeri) Columbiana, and devia, and the rest of the group. 



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

 same as I have described above for Triodopsis. It will be noticed, however, 

 that the marginals (as in M. exoleta and Wetherbyi) do not always have their 

 cutting points bifid. 



The other type of dentition in the subgenus Mesodon is shared by M. thy- 

 roides, clausa, and Wheatleyi. 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, bluntly 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 proportionally developed cutting point, 

 sometimes bifid in the extreme marginals, and usually with a small side cut- 

 ting point. 



As in all the genera of disintegrated Helix, the marginal teeth of Mesodon 

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

 membrane. 



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 genus. The 

 peculiar outer lateral teeth and marginals of 31. thyroides, for instance, would 



1 I regret my inability to review the membranes of all our species to ascertain the re- 

 lations of this bulging to the side cutting point. Those who in future study the subject 

 must pay especial attention to this point. The figures of Semper (Phil. Archip.) are the 

 most satisfactory ever published. 



