226 WORTH-CAROLINA GEOLOGICAL SURVEY. 



Their vertebrae are sometimes preserved, but they must be 

 exceedingly rare in beds which are as loose as the clays and 

 sands of the tertiary deposits. But the teeth, being protect 

 ed by a very dense enamel, and having a firm strong core, re 

 sist change for ages ; it is in these organs, therefore, that 

 memorials of this highly interesting order of fish have been 

 preserved. 



The teeth being attached loosely to a cartilaginous jaw, 

 are almost always separated and detached ; and hence, they 

 occur singly. Of the mode in which they are connected, we 

 are informed by the living species which inhabit the adjacent 

 seas. From this source of information, we may be assured 

 that these single teeth were arranged in several rows in both 

 jaws ; that only a single one, those of the front, stood up 

 right, while the remainder lay flat with the points directed 

 backwards, or obliquely so. &quot;When the front teeth drop out, 

 its place is supplied at once by the uprising of that one which 

 is opposite the vacant space. The teeth, though very numer 

 ous, differ but little in form, though they differ more in size. 

 The most remarkable difference may be observed on compar 

 ing the symphysal teeth, or middle row with those on each 

 side. Thus, Fig. 49, shows a front section of the lower jaw 

 of the galeocerdo ar clients ; the outer row standing upright. 



FIG. 40. 



those behind lying flat, and the middle teeth consisting of a 

 series of small ones. This figure, therefore, is a type by 

 which the reader may compare the prevailing arrangements 

 in the existing, as well as in this extinct family of fishes. 



