242 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.55. 



margin the dental ridge (h). Unless otherwise specified, the outline 

 drawings of the jaws show the jaw lying on its anterior face — that 

 is, as if viewed from point e in figure 1. In this view the entire 

 dental ridge (fig. 2, h-h) and the base of the jaw {g) may be seen. 

 The dental ridge is usually sinuous in outline, following the bases 

 of the several teeth. The base of the jaw may be regular or deeply 

 emarginate. 



In some species of the genus Camharincola the posterior portion of 

 the dental face (fig. 1, d e) may be produced caudad in large speci- 

 mens so that it extends beyond the level of the longest tooth in the 

 jaw, if the jaw is viewed from point e. If viewed from the anterior 

 face (point /), this produced portion of the dental face forms a hood 

 over the teeth and may be seen lying behind them. In figure 17 a 

 small hood of this sort is shown, although this jaw is viewed from 

 the hood side. 



In all of the species examined the jaws, and the dental face in 

 particular, were more or less arcuate. In measuring the jaws this 



fact should be remembered, as the teeth do 

 not lie in a single plane and the jaw may be 

 perceptibly flattened and thereby widened 

 h in an effort to force the teeth into one focal 

 plane. 



The dentition of the jaws has been ex- 

 ^ pressed by two numbers, the first in each 



Fig. 2.— Hypothetical jaw, j> ' ■ j. . i ^ j: < . i 



TYPE 1, AS SEEN FROM POINT ^asc refcmng to the number of teeth, re- 

 E IN FIG. 1. A-c=TEETH, G= gardlcss of sizG Or shape, on the dorsal or 



BASE OF JAW, H-H = DENTAL • i ,, t . .1 1 



jjiPQj, upper ]aw, and the second to the number 



of teeth on the ventral or lower jaw. Con- 

 sidering the relative size and position of the teeth on the jaw several 

 types of jaws were recognized, each of which has been designated by a 

 roman numeral. The primitive type of jaw seems to be one with sev- 

 eral, subequal teeth (fig. 2). The modifications of this type have pro- 

 gressed along either or both of two lines, (1) a reduction in number of 

 teeth, and (2) an increase in the size of certain teeth correlated with 

 the reduction in size of certain other teeth. As the arrangement of the 

 teeth on each jaw in species regularly having a dental formula of 

 5-5 or less could be interpreted in terms of a 5-toothed jaw, the 

 teeth of which are symmetrically placed with reference to the median 

 axis of the jaw, the 5-toothed jaw was cliosen as a standard for 

 the description of jaws. Considering the 5-tcothed jaw as typical, 

 its teeth have been lettered G-B-A-B-C (fig. 2), Capital letters have 

 been used when the teeth were subequal and both capital and small 

 letters when the teeth varied in length, long teeth being denoted by 

 capitals. In the jaws bearing more than five teeth in the occasional 

 individuals of species regularly having a dental formula of 5-5 



