OP CONCHOLOQY. 



47 



NOTE ON THE JAWS OF HELICES. 



BY W. G. BINNEY. 



In examining the jaws of the North American Helices, I 

 have been struck with the want of uniformity in the number 

 and disposition of the ribs on the anterior surface in different 

 individuals of the same species. To call attention to the sub- 

 ject, and guard against too much reliance on the number of 

 ribs as a specific character, I have here figured (plate 6, figs. 2, 

 — 10,) the jaws of nine inviduals taken at random, from a 

 number of Helix tryoni^ Newc, received from Dr. J. G. 

 Cooper. All the individuals were apparently mature. 



I mention this as in his work on the MoUusks of France, 

 Moquin-Tandon remarks that young individuals of the 

 genus Helix have fewer ribs upon the jaw than adult, or aged 

 ones, that the central ribs appear first, their number increaing 

 from the centre towards the extremities, so that the appear- 

 ance of the denticles on the margin of the jaw follows the 

 same order as prevails in the dentition of the superior verte- 

 brates. He also observes that in the case of some species 

 furnished with but few, widely separated ribs, there are 

 accessory, but not very prominent ribs formed in the intervals, 

 when the animal becomes aged. The arrangement of the ribs 

 in the jaws figured does not seem to be governed by these 

 laws. 



Fig. 1 represents the shell of the species from which the 

 jaws were taken. This and the other figures are used by 

 lermission of Prof Henry in anticipation of the Smithsonian 

 pnstitution's work on the Land and Fresh-water Shells of 

 North America. 



