54(3 UNITED STATE8 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Polita, Held. (1837), Isis, 916. 



Vitrea, Fitz. (1833), Verz., 99. 



Hyallniu, Agassiz (1837), Nouv. Me'm. Soc. Helv., I, ap. Charp., 13. 



Lucilla, Lowe (1852), Ann. and Mag. N. H., IX, 115. 



Eiym. — viXivoc, glassy. 

 Examp. — Helix cellaria, Milller. 



Shell small, or attaining a medium size, shining and vitreous, varying 

 from subdiscoidal to depressed-subglobose ; axis perforated, moire or less 

 umbilicated, or with umbilicus closed ; volutions from four to about six, 

 increasing regularly in size, not angular on the periphery, last one not declin- 

 ing at the aperture, which is generally lunate or rounded-sublimate. 



The species of this genus, like those of several allied groups, are gener- 

 ally rather small, and have a thin, shining, vitreous shell, with a thin, simple 

 lip, and a more or less rounded-lunate aperture. Like many other land- 

 mollusks, the existing species are found in damp, shady places. 



Very widely different opinions exist in regard to the limits of this genus, 

 and the groups that ought or ought not to be included as sections or sub- 

 genera under it. Mr. Tryon, who has published a valuable monograph of the 

 Terrestrial Mollusca of the United States, only admits two sections among 

 our species. These are Hyalina proper, and Omphalina, Rafinesque ; while 

 some other authors place the latter group as a section of the distinct genus 

 Zonites, Mont fort. 



Mr. Binney, however, who has also devoted much attention to the study 

 of the North American land-shells, includes as subgenera, under the genu-; 

 Hyalina, the following groups among our species : Hyalina proper, Mesom- 

 phix, Raf, Conulus, Fitzinger and Gastrodonta, Albers ; while Mr. Tryon and 

 some others range Mesomphix as a distinct genus, including under it Conulus 

 and Gastrodonta as subgenera. Professor Morse has also (as I would think 

 correctly) separated two of the species included by Mr. Binney as types 

 of new genera. On the other hand, Dr. Albers, a distinguished German 

 Conchologist, not only includes, as sections of this genus, Hyalina proper, 

 Mesomphix, and Gastrodonta, but three other sections (Morchia and Edusa, 

 Albers, and Ammonceras, Pfeiffer). 



1 have not studied the Helicillid/e, however, with sufficient care to pre- 

 tend to decide where others with far better facilities have differed so widely ; 

 but, judging from all analogy, I am rather inclined to concur with those who 

 restrict the genera to the narrowest limits. 



