18 PALEONTOLOGY OF OHIO. 



which mark their posterior lateral angles. Every anatomist will recog- 

 nize the probability that these furrows served for the reception of nerves 

 and blood vessels which passed to the posterior members. When, now, 

 we compare this shield with corresponding or analogous parts in other 

 animals, we find some remarkable and suggestive resemblances : 



1st. The elements of the ventral shield of Dinichthys offer a striking 

 parallel with the flattened bones which compose the lower parts of the 

 pectoral and pelvic arches in Plesiosaurus, viz., the coracoids, the inter- 

 clavicular (or sternal) cartilage, and the pubic bones. It is quite certain 

 that the bones of each series held corresponding positions and performed, 

 more or less perfectly, the same functions, and it has seemed to me not 

 impossible that they are homologous. In this view the anterior lateral 

 bones of the shield would be considered as coracoids, the posterior pair as 

 pubic bones, and the median plate as the equivalent of a sternum, and, 

 perhaps, an interclavicle. 



The notion that the paired bones of the plastron of Diniohtlxys can be 

 the representatives of the coracoids and pubic bones of reptiles will at 

 first sight appear so heterodox as hardly to deserve a second thought, and 

 the view that they are simply dermal ossifications, like the scutes of the 

 sturgeon, the dorsal plates of the Siluroids, etc., will seem much more sim- 

 ple and satisfactory. 



It would certainly be an easy way of explaining the origin of these 

 plates, to suppose them to form one of the almost infinitely varied phases 

 assumed by the exoskeleton , of fishes, but it often happens that the easy 

 and simple explanations of Nature's problems are not the true ones, and, 

 as will be shown further on, reasons may be found for seriously doubting 

 that these bones form any part of the exoskeleton. 



2d. In the turtles the under side of the body is defended by a plastron 

 which performs the same functions and resembles much in character the 

 ventral shield of Dinichthys. Tet it will be noticed that there are im- 

 portant differences between them. The plastron of the Chelouians usual- 

 ly consists of four pairs of plates, with a wedge-shaped intermediate one 

 in front. All of these are claimed, and are apparently shown, by Rathke, 

 to be membrane bones developed in the integuments, and having no con- 

 nection with the endoskeleton. The anterior three bones of the plastron 

 of the turtle are thought by Huxley to correspond to the three gular 

 plates of the Labyrinthodont Amphibians and to be the representatives of 

 the clavicles and an interclavicle. 



The two pairs of bones which compose the central and chief portion of 

 the plastron of the turtle, hold the positions of the two pairs in the shield 

 oi Dinichthys, and some turtles possessthose which are not greatly unlike 

 them in form. It is, therefore, not impossible that they are their equiva- 



