46 A MANUAL OF AMERICAN LAND SHELLS. 



and creuellatiug the lower margiu. The ribs are ofteu almost obsolete, 

 or represented by wrinkles or coarse striae. They are present on the 

 anterior surface of the jaw only, or on both anterior and posterior sur- 

 faces. They are distant, narrow, stout, few ; or crowded, broad, stout, 

 and numerous. Their number is within certain limits inconstant in the 

 same species. They sometimes are very broad, and seem like separate 

 plates soldered to the anterior surface of the jaw, or to be formed by a 

 folding of the jaw upon itself. When this appearance of folding into 

 plates is given, it will generally be found that the plait-like sections are 

 actually separated by distinct but delicate ribs. When this form of 

 ribs is found, they are either vertical or inclined obliquely towards the 

 median line of the jaw. Sometimes this last arrangement is developed 

 to such a degree that the delicate ribs meet before reaching the bottom 

 of the jaw, and a triangular compartment is left at the upper center 

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

 and membranous. 



When the jaw is striated and not ribbed, the striae are vertical, or 

 they converge towards the median line. There are often transverse 

 striae also, and transverse lines of re-enforcement. 



The upper margin of the jaw is often extended into a stout membra- 

 nous attachment, apparently of the same material and consistency as 

 the jaw itself, and showing the same continuity of structure by the 

 striae of the jaw extending into it without interruption. This is not 

 the accessory quadrate plate mentioned above. 



The jaw is found in every degree of consistency, from very thick to 

 quite membranous and almost transparent. 



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. 



It will be seen below that these i)eculiarities of the jaw, taken in con- 

 nection with the characters of the lingual membrane, have till now 

 appeared to furnish reliable characters for classification. It must be 

 confessed, however, that exceptions to the usual constancy of char- 

 acters have been noticed in some genera ; sometimes the difference be- 

 tween striae and ribs is difficult to determine ; sometimes the beak-like 

 prominence is greatly modified by a simple median projection. In some 



