NATURAL SCIENCES OF PHILADELPHIA. 143 



surface of the jaw only, or on both anterior and posterior surfaces. 

 They are distant, narrow, stout, few (fig. 8) ; or crowded, broad, 

 stout, and numerous (fig. 14). 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 sur- 

 face of the jaw, or to be formed by a folding of the jaw upon itself 

 (fig. 12). When this appearance of folding into plates is given, 

 it will generally be found that the plait-like sections are actually 

 separated b}' 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 de- 

 veloped to such a degree that the delicate ribs meet before reach- 

 ing the bottom of the jaw, and a triangular compartment is left 

 at the upper centre of the jaw, its base being upward (fig. p. ). 

 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 (fig. p. ). There are 

 often transverse strias also, and transverse lines of re-enforcement 

 (pi. XVI, fig. 3). 



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

 membranous attachment, apparently of the same material and 

 consistency as the jaw itself, and showing the same continuity of 

 structure by the stride of the jaw extending into it without inter- 

 ruption. 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 den- 

 ticulated. It is simply concaA r e, or furnished with a more or less 

 developed beak-like median projection. 



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

 horseshoe-shaped, short, high. 



It will be seen below that these peculiarities of the jaw, taken 

 in connection with the characters of the lingual membrane, appear 

 to furnish reliable characters for classification. 



