NATURAL SCIENCES OF PHILADELPHIA. 145 
23123 teeth, that is, 23 teeth on each side of the central tooth, 
making 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 pl. III., 
fig. 12, a, b, ec, 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’ 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 f f. ‘These last are sub- 
obsolete in the species figured, but in figure 7 a of the same plate 
these side cusps are more fully developed. All the cusps are in 
most cases surmounted by distinct cutting points ; 7 is the median 
cutting point, g g the sede 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 (pl. XVIL., fig. 3 6), 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 T use the term upper and lower to describe the figure I give of the 
base of attachment. More properly I should say postertor and anterior to 
describe their position on the membrane. 
