142 DR. GIBBES' MONOGRAPH OF THE 



upper and lower jaw, being straighter and more acute in one than in the other. 

 When the front teeth are similar to those at the sides, they are usually smaller and 

 more pointed, and at the symphysis in both jaws often there is a small tooth, or several 

 of a peculiar form. 



The teeth are also characterized by serratures varying in size, and by small lateral 

 denticles, which are not always present in young teeth and do not form specific 

 distinctions, though in adults assist in referring them to species. 



Professor Agassiz has pointed out a distinction between Carcharias and 

 Carcharodon, in the dentine of the former presenting a hollow cone, while in the latter 

 it is solid, the only character they have in common being their microscopic structure. 

 As the latter genus is one of the most prolific in species and prominent in size and 

 form, 1 commence the series with it. 



Genus CARCHARODON, Smith. 

 SPECIES. 



1. C. MEGALODON, Agass. 



Var. RECTIDENS, Agass. 



" SUBA.URICULATUS, AgttSS. 



2. C. ANGUSTIDENS, AgUSS. 



Var. LANCEOLATUS, Agass. 

 " HETERODON, Agass. 

 " MEGALOTis, Agass. 

 " AURicuLATUS, Agass. 

 *' TURGiDUS, Agass. 

 " sEMiSERRATUS, Agass. 

 " TOLiAPiciJS, Agass. 



3. C. ACUTiDENs, Gibhes. 



4. C. MORTONi, Gibhes. 



5. C. LANCiFORMis, Gihhes. 



6. C. suLCiDENS, Agass. 



The general form of the teeth of Carcharodon is that of an isosceles triangle, those 

 in the upper jaw being usually a little larger than in the lower, and not as dissimilar 

 as the upper and lower teeth of Carcharias. At the symphysis they are almost 

 entirely straight ; the next have their edges sloping, and the last are almost without 

 the middle cone. In the lower jaw they are pointed and sloped similarly on both 

 edges ; but what distinguishes them particularly from those of the upper jaw, is a 

 very perceptible furrow in the enamel at the base of the crown. The whole 

 circumference of the cutting edges is covered with fine notches, (dentelures) very 



