NATURAL SCIENCES OP PHILADELPHIA. 145 



23 1 23 teeth, that is, 23 teeth on each side of the central tooth, 

 m akin sr 47 teeth in the entire transverse row. 



The characters of the individual teeth vary greatly in the 

 various genera, especially in some of the genera foreign to our 

 limits. In most cases, however, there are two distinct types of 

 teeth, the quadrate and aculeate. The former is shown in pi. III., 

 fig. 12, a, 6, c, d, is the portion of the tooth which rests upon the 

 membrane ; I have called it the base of attachment. It varies in 

 its proportional length, and in the greater or less expansion of 

 the lower 1 lateral angles. The upper margin of this base of at- 

 tachment is broadly reflected ; e marks the reflected portion, which 

 I term the reflection. It is usually tricuspid, the median cusp h 

 being much longer than the side cusps ff. These last are sub- 

 obsolete in the species figured, but in figure 1 a of the same plate 

 these side cusps are more fully developed. All the cusps are in 

 most cases surmounted by distinct cutting points ; % is the median 

 cutting point, g g the side cutting points. These cutting points 

 are not always present on the side cusps, and, even when present, 

 are sometimes not readily detected. Indeed, this is the most diffi- 

 cult point of study of the whole membrane. The cusps and cut- 

 ting points vary in development in the various species, and some- 

 what so in different portions of the same membrane. 



The other type of tooth (pi. XVII., fig. 3 b), which I call acu- 

 leate, differs in not having a quadrate base of attachment, but 

 usually one of a somewhat sole-like form. Its upper margin is 

 not reflected, but from its whole surface springs a single large 

 cutting point, usually thorn-shaped, but sometimes more spine- 

 shaped. The apex of the cutting point is sometimes bifid, or even 

 trifid, even in the same genus. 



Of these two types, quadrate and aculeate are all the teeth now 

 known. Of the quadrate type many and dissimilar forms are 

 known, but all have the quadrate base of attachment. 



The characteristics of central, lateral, and marginal teeth are 

 given under each genus or subgenus. 



1 I use the term upper and lower to describe the figure I give of the 

 base of attachment. More properly I should say posterior and anterior to 

 describe their position on the membrane. 



