MANUAL OF CONCHOLOGY. 



FAMILY ZONITIDM. 



Animal protected by a spiral, heliciform shell, within which 

 it is completely retractile ; tail provided with a caudal mucus- 

 pore; jaw with a median inferior projection, not ribbed; the 

 central tooth in the lingual ribbon is tricuspid, with the middle 

 cusp long and narrow, laterals bicuspid, marginals acuminate, 

 unicuspidate or bicuspidate. 



Shell usually heliciform and perforate or umbilicate, often 

 thin, and more or less transparent, the lip of the aperture sim- 

 ple, neither reflected nor with thickened margin. 



The family thus defined includes several hundred species, 

 usually considered Helices, but which differ from that group in 

 the generally thinner, more translucent shells, the aperture with 

 simple margin ; the genuine Helices being generally thicker, and 

 with the margin of the aperture either reflected or thickened within . 



The jaw also is not ribbed, unlike most Helices, and the den- 

 tition differs. Externally, the animal is at once distinguished 

 from Helix by the presence of a mucus-pore. There are some 

 pulmoniferous mollusks having Zonitoid shells, whilst their soft 

 parts place them in Helicidse, and vice versa. The animals of 

 most of the species, however, are entirely unknown to science, 

 and we are thus reduced to analogies of the shell in determining 

 their systematic position. On the side of the Vitrinidae the re- 

 lations of both animal and shell are quite as intimate, the group 

 Otesia, for instance, which at the last moment I decided to in- 

 clude in the latter family, being- made a member of the Zonitidae 

 by Pfeiffer, Fischer and other good authorities. The Vitrinid 

 mantle-lobes exist in a modified form in some of the groups of 

 the present family. 



