12 PALEONTOLOGY OF OHIO. 



The remarkable correspondence between the ventral shields of Dinich- 

 thys and Coccosteus will probably be accepted without argument as evi- 

 dence of consanguinity, even though the cephalic and dorsal shields 

 should seem quite unlike. In fact, however, the differences in the arma- 

 ment of the upper part of the body of the two genera are those of degree 

 rather than of kind, and are much less important than would appear at 

 first sight. They will be referred to again on another page. 



The relationship of Dvnichthys and Coccosteus is also indicated by 

 similarities of dentition. The jaws and teeth of Coccosteus are not yet 

 perfectly known, but Pander lias figured the mandibles, and has given 

 some indication of the dentition of the upper jaw ; or at least has given 

 figures which show that the premaxillaries (?) were united in one piece, 

 which was set with small, pointed teeth. In all the specimens, figured and 

 described by Pander, the anterior extremities of the mandibles seem to 

 have been defective, and we have yet to learn precisely how they were 

 united. In the middle portion of each mandible there is seen a row of 

 teeth formed by the prolongation of the jaw-tissue, precisely as in Dinich- 

 thys Hertzeri. In the specimens represented in the figure cited above, 

 in addition to the view given of the under side of the ventral plates, the 

 posterior extremities of both mandibles are distinctly shown. These are 

 flattened and spatulate, precisely as in Dinichthys. Unfortunately the 

 anterior extremities are broken away ; the mutilation of the specimen 

 depriving us of information that would be pechliarly precious in this 

 connection. Many other fragments of the mandibles of Coccosteus are 

 figured by Pander, but 'none supply us the data necessary for a complete 

 compq-rison with those of Dmichthys. We are, however, justified in 

 saying that the dentition, as a whole, was very similar in the two genera. 



When now we come to consider the bones of the cranium and the dor- 

 sal shields, we find some marked difEerences between those of Coccosteus 

 and Dinichthys. In Coccosteus the cranial bones form a nearly circular, 

 solidly cemented, and highly ornamented cephalic shield, to which the 

 dorsal carapace is supposed to have been firmly united. In regard to 

 this latter point there is some doubt, for the articulation of the " supra- 

 scapular bones" with the posterior lateral angles of the head is such as 

 to indicate considerable freedom of motion ; and I am led to believe that 

 the body plates were disconnected with the head except by this articula- 



resemblanoe to the coracoids of Plesiosaurus. In the present state of our knowledge it 

 would doubtless he safer to consider these resemblances those of anolagous rather than 

 homologous bones, but for reasons which will he given further on, it seems to me not 

 impossible that we have in the plastron of Dinichthys elements of both the pectoral and 

 pelvic arches, here more highly developed than in any living fishes, and presenting 

 Amphibian and even Reptilian characters. 



