296 A MANUAL OF AMERICAN LAND SHELLS. 



readily mistaken for a distinct side cutting point, and indeed replaces 

 it.* The outer laterals, however, in most of the species have a much 

 more developed side cusp than the inner laterals, bearing a well-devel- 

 oped cutting lioint (Fig. A, Fig. 16), but not all the species, as some 

 have no well- developed side cusp and cutting point on their outer lat- 

 erals, nor does it appear except on the decided marginals. It is thus 

 in 31. 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 

 Mesodon. In M. exoletus the cutting point remains the same, and also 

 in Sayii, profundus, Wetherhyi, and MitcheUianus, but in elevatus the 

 transition teeth are characterized by the bifurcation of the large cut- 

 ting j)oint; the same occurs in albolahris, mnltilineatus, Roemeri, Golum- 

 bianus, and devius, and the rest of the group. The general character 

 of the teeth in this section of Mesodon is about the same as I have de- 

 scribed above for Triodopsis. It will be noticed, however, that the 

 marginals (as in M. exoletus and Wetherhyi) do not always have their 

 cutting points bifid. 



The other type of dentition in the genus 3l€sodon is shared by M. 

 thyroides, clausus, Andrewsi, and Wheatleyi. The centrals and first lat- 

 erals have subobsolete side cus])s, without cutting points. The outer 

 laterals have no side cusps, but retain the type of the first laterals; they 

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

 bluntly pointed cutting j^oint, 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, some- 

 times 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 deuticulation, even in most cases 

 on the same membrane. 



The study of the dentition of 3Iesodon shows that we must be pre- 

 pared to find considerable variation in thecharacter of the teeth of any 

 genus. The peculiar outer lateral teeth and marginals of M. thyroides, 

 for instance, would hardly have been expected, so utterly different are 

 they from those of albolahris. Again, we should hardly have expected 



* I regret my inability to review the membranes of all our f-pecies to ascertain the 

 relations of this buloing 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. 



