246 PROCEEDINGS OF THE ACADEMY OF [1893. 



SPECIAL DESCRIPTIONS. 



Since the number of species has increased considerably in the last 

 decades, they should be described as exactly as possible, especially in 

 view of the above facts. Thus I think allowable the alteration of 

 previous descriptions of several species, especially V. pulchella and 

 costata, in some points which proved to be essential in exactly 

 characterizing them ; and there is no use in citing the older 

 descriptions, 11 or in especially mentioning the changes made — the 

 former are found elsewhere, and a comparison will show the 

 latter at once. 



In spite of some striking differences in the formation of the shell, 

 it appears to be impossible to divide the species under well-defined 

 subgenera or sections, as there are intermediate forms everywhere, 

 and the anatomy, especially of the jaw and radula, furnishes no 

 reason for doing so. An attempt, however, has been made to 

 arrange them in natural groups, in the table after the special 

 descriptions. 



It might be thought useless to name many varieties ; but this 

 could hardly be avoided after careful study of the forms, and more 

 of this has been done among larger mollusca. 



Judging from Morse's and W. G. Binney's description of Patula 

 (Planogyra) asteriscus Mse., I supposed this species might be a 

 Vallonia. One specimen was not softened sufficiently to be exam- 

 ined thoroughly. The radula, however, showed the essential char- 

 acters of Vallonia. Fresh specimens must be studied. Yet the 

 finer surface structure of the shell is different from that of all true 

 Vallonia I have seen : the nucleus is thickly set with minute pits 

 somewhat elongated radially, and the post-embryonal whorls show 

 microscopic revolving lines, as figured by Morse (I. c). To judge 

 from Morse's (I. c.) figure 51. p. 24, of the front part of the animal, 

 this would also not agree with that of Vallonia. 



V. pulchella Muller. PI. VIII, fig. A, G. H, I, K, L. Man. Conch., PI. :J2, fig. 15. 1 2 



Hel. pulchella Muller, Verm. Hist., II, 1774, p. 30. 



Hel. pulchella Auct. 



Hel. paludosa Da Costa, 1780- 



Hel. crystallina Dillwyn, 1817. 



11 It may be mentioned that they do not always agree among themselves, and 

 that different forms have been comprised under one name and different characters 

 regarded as essential or otherwise by various authors. 



12 Plates 32 and 33, referred to throughout this paper, will be found in Tryon 

 and Pilsbry's Manual of Conchology (2), VIII. 



