ANNELID GENUS CAMBARINCOLA — HOFFMAN 283 



primitive condition involves subequal jaAvs in which the cusps are 

 simihir in size und shape, and that speciahzation influences enhirge- 

 ment of the median tooth of the upper jaw. He devised a system 

 of nomenclature to reflect size order of the teeth, and is responsible 

 for the term "dental formula" by which the number of teeth can be 

 stated numericaUy (e.g., 5-4), indicating that the upper jaw has 

 a median tooth and two more on each side while the lower jaw has 

 two paramedian teeth plus one lateral cusp on each side. However, 

 other arrangements are common, particularly in Oriental species. 

 The American Ceratodrilus has small transverse jaws each with 6 

 to 8 equal dentations, and genera such as Xironogiton are likewise 

 small-jawed but usually with a smaller number of teeth. Ellis studied 

 variation m the jaws of what he considered to be Cambarincola 

 ■philadelphica and found considerable individual variation, although 

 it is now known that he had confused several species under that name. 



Insofar as Cambarincola is concerned, there is considerable latitude 

 in the ranges of variation, individual and geographic as well as specific. 

 Individual variation is largely a function of age, with small specimens 

 of a species having proportionately larger jaws in relation to head 

 size, with more acute dentations. Older specimens tend to have the 

 lateral cusps obscured or worn away. Geographic variation affects 

 the relative length of the jaws in such species as Cambarincola vitrea. 



The typical jaw arrangement in the genus is one of essentially 

 equal-sized jaws, the dorsal with a large median tooth and two 

 or four small lateral cusps, and the ventral jaw with two large 

 paramedian terminal teeth and a pair of small lateral dentations. 

 Various departures from this basic pattern occur, however, afl^ecting 

 both the relative size and dental formulae, and a rough classification 

 can be drawn up as follows: 



Jaws isomorphic, having the same outline in dorsal aspect, and the dental formula 

 5-5 or 3-3. In lateral aspect, the jaws are mirror images. The species 

 so characterized can be called "homognathous," and they occur sporadically 

 in different sections of the genus. C. fallax is a good example. 

 Jaws anisomorphic, being dissimilar in armature, the dental formula being 

 5-4 or some other combination of odd and even numbers. This general 

 class is further divisible according to the relative size and shape of the teeth. 

 Teeth virtually subequal, presumably the primitive condition, occurring in 



C. vitrea and some related forms. Such species are "homodontous." 

 Teeth dissimilar, the median dorsal and paramedian ventral dentations enlarged, 

 this being the most typical condition in the genus, and can once more 

 be divided according to the relative size of the jaws: 

 Jaws similar in size in both lateral and dorsal aspects, this including the 

 majority of species such as C. philadelphica, macrodonta, ingens, etc. 

 Jaws markedly dissimilar in size, the dorsal jaw up to four or five times as 

 bulky as the ventral. This is presumably an advanced stage of speciali- 

 zation, and species so characterized may be called "heterognathous." 



