320 PALAEONTOLOGY OF ILLINOIS. 



Assuming- that the Carboniferous fish was of similar shape, which seems 

 highly probable, that is remarkable for the breadth of the anterior or 

 cephalic region, the mandibles of the ancient fish, which were near 

 twenty-eight inches in length, would indicate a total length of from 

 fifteen to twenty feet — truly formidable dimensions compared with 

 the largest known individuals of the existing Gestraeeonts.- 



The teeth of the posterior rows are generally distinguishable by their 

 more robust proportions and the rounded extremities. Those of the 

 anterior rows are appreciably more slender, their extremities more 

 angularly terminated. Only the teeth of the median row are promi- 

 nently acuminate, though the first contiguous rows either side present 

 a partially developed eccentric cone. As seen from above, the teeth 

 are slightly sigmoidally curved, those of the median row with the arch 

 backward, while those of the lateral rows are slighly arched forward. 

 In the relative number of buttresses in the anterior face there is marked 

 irregularity, the buttresses increasing in number in the same row with 

 age, at the same time subject to numerical modification by accidental 

 causes ; yet the extreme posterior rows appear to possess a relatively 

 larger number of buttresses than the teeth of the corresponding rows 

 in front, while there would also appear to be an irregular diminution in 

 the number in the successive rows from the median row towards either 

 extremity of the ramus. The same irregularity obtains in the dispo- 

 sition of the buttresses of contiguous teeth in the same row, so that 

 no two individuals present precisely the same details of superficial con- 

 formation, although in general proportions and outline the most marked 

 persistency prevails, which may also be applied to the lesser details of 

 ornamentation. 



The examples given in illustration of the various forms of teeth from 

 different portions of the ramus at the same time exhibit the details of 

 modification presented by the various individuals. We have included 

 therein certain specimens found with the group of teeth discovered by 

 Prof. Todd, which it is supposed probably belonged to the upper jaw 

 These latter are represented by teeth which differ in certain particulars 

 to such an extent as to present strong contrast to the teeth of the 

 opposite jaw, as determined by comparison with the great mandibular 

 specimen from Kansas. The teeth here especially referred to are repre- 

 sented in PI. S, fig. 6 and 7. The former, on account of its very eccen- 

 tric median cone, is not comparable to the teeth constituting the median 

 row of the lower jaw, which, as shown by comparing the Iowa and 

 Kansas specimens, possess marked persistency in form and proportions ; 

 it would therefore seem more probable that we have here a tooth which 

 occupied a corresponding position upon the right ramus of the upper 

 jaw. The latter specimen, fig. 7, also presents characters which appear 



