f 

 62 PALEONTOLOGY OF OHIO. 



Carboniferous species yet known, were without scales, but the same difier- 

 ence probably existed between the Geratodus of the Trias, and the " Bar- 

 ramunda " of Australia ; both of which have been included by Dr. Gunthcr 

 in the same genus. No scales have been found in connection with the 

 Triassic teeth, while the Australian Ceratodus has the body covered with 

 large imbricated scales. 



Fig. 17 represents the tooth of the natural size, seen from above. 

 Fig. 17 a, a side view of the same, and Fig. 175, a profile view of the 

 marginal row of denticles. 



■"jjL Genus Heliodxjs (nov. gen.). 



A Ganoid fish, closely allied to Dipterus, from which it differs in hav- 

 ing the upper palate teeth united together to form a single large rounded, 

 or semi-circular triturating plate, bearing several tuberculated ridges 

 which radiate symmetrically on either side of the central line. These 

 ridges are highest on the margin of the tooth and gradually diminish to 

 the centre, which is smooth. The dental plates of the lower jaw have 

 not yet been found. Whether they were consolidated in one, or sepa- 

 rated like those of Dipterus is not yet known. Since, however, they 

 were in Dipterus and Ctenodus more M'idely separated than the upper 

 teeth it is possible that they were not joined together in Heliodus. In 

 microscopic structure this tooth is similar to those of Dipterus. The 

 tuberculation of the ridges is precisely the same, and if divided through 

 the middle, each half would be accepted as one of the upper dental 

 plates of that fish. 



There is little doubt that we have in Heliodus a new member of the 

 family of Dipterine Ganoids to which Dipterus, Ctenodus and Geratodus 

 belong, and its dental plates present a simple but hitherto unknown- modi- 

 fication of the characteristic dentition of the group. In the other genera 

 of the family the palate teeth vary much in form and in the number and 

 character of their radiating ridges. In Dipterus the upper teeth have 

 the form of right-angled triangles, or half opened fans, the ridges being 

 set with rounded and generally obtuse tubercles. The lower teeth are 

 longer, like a fan two thirds opened. In Gtenodus the number and form 

 of the teeth is the same as in Dipterus, but the radiating ridges are 

 generally more compressed, and the tubercles, are more acute. In Cera^ 

 todue the teeth are smooth, the ridges few and large, and without tuber- 

 cles. Finally in Heliodus we have the upper pair of palate teeth firmly 

 joined in one plate, taking the form of a fully opened fan, and bearing 

 radiating tuberculated ridges like those of Dipterus. 



