166 071 the Systematic Arrangement of 



delicate ribs. Wlien by this form of ribs the jaw is divided 

 into separate compartments, these compartments or plates 

 are either vertical or inclined obliquely towards the median 

 line of the jaw. Sometimes this last arrangement is de- 

 veloped to such a degree that the oblique lines of separation, 

 or delicate ribs, of the plates meet before reaching the bottom 

 of the jaw, and a triangular plate is left at the upper centre 

 of the jaw, its base being upward. This form of jaw is 

 usually thin and membranous. It has been considered the 

 characteristic of the subfamily Orthalicince, or of the Goni- 

 ognatha by most authors. We, however, can treat it only 

 as a modification of the usual form of ribbed jaw, inasmuch 

 as we find it in various degrees of development in Bulitnidus, 

 Bulimus, and even in Helix.* It will be seen below that we 

 restrict the Ortlialieinod to those genera whose jaw is in actu- 

 ally free, imbricated pieces. 



When the jaw is striated and not ribbed, the strife are 

 vertical, or they converge towards the median line, as do the 

 plates in Macroceramus and Cylindrella. There are often 

 transverse strife also. 



The upper margin of the jaw is often extended into a 

 stout membranous attachment, apparently of the same mate- 

 rial and consistency as the jaw itself, and showing the same 

 continuity of structure by the striaj of the jaw extending into 

 it without interruption. 



The jaw is found in every degree of consistency, very 

 thick in most species of Helix, quite membranous and almost 

 transparent in some of Bidimulus, in Macrocerainus and 

 Gylindi'ella. 



The cutting margin of the jaw is smooth, crenellated, or 

 denticulated. It is simply concave, or furnished with a more 

 or less developed beak-like median projection. 



In shape the jaw ranges from scarcely arcuate, long, low, 

 to horse-shoe shaped, short, high. 



* See our notes on Helix turhiniformis in Ann. N. Y. Lye. x, pi, ii, fig. 2. 



