44 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



accessory quadrate piece attached to its upper margin ; or in separate, 

 detached pieces, free on their lower edges, usually soldered together 

 into one single piece ahove. It differs also in being with or without a 

 median beak-like projection to its cutting edge ; also in its ends being 

 more or less acuminated; but still more by the presence or absence of 

 striae or rib-like processes on its anterior surface. When present, the 

 ribs are found in every degree of development, passing quite across the 

 jaw and denticulating one or both margins, or only developed on the 

 lower portion of the jaw, and crenellating the lower margin. The ribs are 

 often almost obsolete, or represented by wrinkles or coarse stria?. They 

 are present on the anterior surface of the jaw only, or on both anterior 

 and posterior surfaces. 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 deli- 

 cate 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 compart- 

 ment is left at the upper centre 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 reinforcement. 



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. 



