206 PROCEEDINGS OF THE ACADEMY OF [Feb., 



ceedingly rare individuals of species which normally have rows of 

 independent teeth acquire the radial barriers of forms more advanced 

 in the scale of evolution. This is exemplified by V. muUidentata, dis- 

 cussed in a previous paper/ and V. andrewsce, mentioned below. 



All this merely goes to show that Paravitrea is in a condition of rapid 

 Teadaptation. Mutation takes place chiefly in two definite directions : 

 (1) Toward progressively earlier loss of internal armature, which cul- 

 minates in toothless species, and (2) the jormation of transverse barriers 

 by coalescence of the teeth. 



Under these circumstances, the path of the systematist is an excep- 

 tionally thorny one, although to the evolutionist the group is full of 

 interesting suggestions. No rigorous definitions are in order; and the 

 distinction between '^species" and "subspecies" — always more or 

 less indefinite — becomes largely a subjective one when large series 

 of specimens are studied. Only a series of individuals representing the 

 whole life cycle, from youth to old age, can give one an adequate idea 

 of the characteristics of the special form from any given locality. 



Through the kindness of Mr. Ferriss and others, I have been able to 

 study large series of most of the species, in addition to the collection of 

 the Academy. 



Aside from Mr. Binney's volumes, which contain merely descriptions 

 of part of the species discussed below, the strongest paper dealing with 

 them is Dr. V. Sterki's Notes on Zojiitidce? The fact that Zonites 

 andrewsi and the Eastern form commonly known as significans have 

 few teeth or none when adult is set forth, and the author shrewdly 

 suggests that these two forms may be the young of Z. placentulus and 

 capsella respectively. Dr. Sterki worked from small series of specimens 

 from a few localities, and with much more ample material it is easy 

 now to criticise his solution of the difficult problem; but he saw fur- 

 ther into the relationships of these forms than any wi-iter up to that 

 time. My former course in separating the species into two sections, 

 one of which {Taxeodonta) was placed under Gastrodonta, was a step 

 backward. No such arrangement would have been entertained had I 

 examined the dentition. In Gastrodonta the lateral teeth are more 

 numerous than in Vitrea or Paravitrea, and conspicuously different in 

 shape. 



' These Proceedings for 1900, p. 145. 



* Nautilus, VII, pp. 13-17, June, 1893. It may be as well to note here that 

 the Gastrodonta described by him under No. 3 (p. 14) is what I described as 

 G. collisella. His No. 6 (p. 15) is apparently G. coelaxis. 



