374 AMERICAN SPECIES OF VERTIGO. 



(6) Above the last, near the suture, there is a small but distinct denti- 

 cle in most examples of V. antirertigo, and it is sometimes seen in speci- 

 mens of V. pentodon from Texas. 



It has already been stated that in general the accessory lamellne are 

 smaller than the primaries, especially in V. antivertigo and V. ovata, 

 as the former 1. La, 2, 3, -I, 5, 6 show in Figs. 1,5, when compared with 

 the latter A, B, C, D, E. But in certain species the differences in size 

 are not so striking, c. g, in many specimens of V. pentodon Say and V. 

 curvidens Grid., where the principals are not at all conspicuously large; 

 here sometimes there is a row of fifteen to seven nearly equal lamella? 

 from the base to the suture (see Fig. 2), so that D and E are scarcely 

 or not at all recognizable by their size, but only by their positious and 

 the fact that they are constant, well developed in specimens where the 

 others are wanting. Besides, D is always a trifle larger, inward, and 

 there is on the outside a nearly corresponding impression or at least a 

 rough line. 



From the description and the table it is evident that there is a line, 

 I venture to say of development, among the species of Vertigo from the 

 simpler to the complicated — from the lower to the higher. The former 

 are represented by those species with simple configuration of last whorl 

 and aperture, as already pointed out, with few and small lamellae, as in 

 V. alpestris and V. ronnebyensis; then in V . tridentata ; from these we 

 come to forms gradually differentiated and more complicated till we 

 reach the end of the series in V. antivertigo and V. ovata, in which all 

 the features above described are conspicuously developed ; the crest 

 and depression iu the last whorl, the expanded margin, the projecting 

 and at the same time impressed angle of the peristome, callosity, and the 

 number and size of the lamella?, (compare figs. 6, 9). 



In the following table I give a few examples of parallel or nearly par- 

 allel species from both continents. 



EUROPE. 



r. antivertigo Drap. 

 V. 8ubstriata Jeffr. 

 J", pygmcea 1 >rap. 

 /'. lilljeborgii \Vest. 

 V. alpestris Alder. 



NORTH AMERICA. 



r. ovata Say. 



V. gouldii Binu. 



V. indesc. (Mass., Ohio). 



V. tridentata Wolf. 



None 1 



Better knowledge of our North American fauna will probably bring 

 to light more such corresponding forms. 



It is evident at once that those forms of Vertigo with cylindrical 

 shape, slowly increasing whorls, relatively small and simple aperture, 

 e. g.. V. alpestris, stand nearest to some Isthmias, and that both of these 

 groups origin ited and differentiated from a common root. In North 

 America the types of Vertigo in general are about the same as in 

 Europe, although the species differ, and although a smaller number has 

 been described so tar. It is a noticeable fact that there are on our 

 continent no species of Isthmia, but it must also be added that no form 

 like V. alpestris has as yet been found here. 



