74 



THE CHRONOLOGICAL DISTRIBUTION 



specimen on which the genus rests is supposed to have been derived from the Devonian 

 rocks of Kentucky, but may have come, from the Subcarboniferous. Dr. Eastman 

 regards this fossil as a true my ; and it certainly possesses many resemblances to one. 



Morphologically the rays are not greatly different from the sharks. Dr. Jaekel* 

 holds that the essential character of the rays is found in the attachment of the pectoral 

 tin-supports to the side of Liu; head, all other characters haying resulted either from this 

 forward extension and anchorage of the fins to the side of the head or from the mode 

 of life affected by the rays. lie doubts, however, the unity of the group which includes 

 the rays, Batoidei of writers. Be is inclined to hold that the Dasyatides (Trygonidee) 

 and the Myliobatidse, the two families forming his " Centrobatidee " (Masticura, Gill, 

 1872), have sprung from forms tike Ptychodw and Strophodus ; and in a, recent publica- 

 tion,-]- the line is traced back to the Psephodonthhe and thence to the Holocephali. The 

 other division of (he rays, Jaekel's Rhinoraji (Pachyura, (Jill, 1872), are supposed to 

 have originated from the Spinacida3 (Squalidse). The two groups are thus removed far 

 apart. I believe, however, that we can say that we have no trustworthy record of the; 

 Masticura before the Cretaceous, when at least Rhombodus and the Ptychodontidee lived ; 

 nor of tin; Pachyura, before the Jurassic, when Rhinobatis, the most primitive of the 

 group, first appears to us. 



Now, on the page of this history of the rays, so modern in comparison with the great 

 age of the class lOlasmobranehii, we art; confronted with tin; proposition that r /'<ui/!o/><t/ix 

 of the Devonian is a true ray. If the divergence of the Batoidei or either division of 

 them occurred so near the root of the; Elasmobranchian stem as the Devonian, they 

 ought to be dignified with the rank of superorder or order, instead of being grudgingly 

 given tin; rank of suborder, as is done by most writers. We must, then, have very 

 satisfactory evidence before we can admit Tamiobatis among the Batoidei. I do not 

 believe that it has been produced. The three characters relied on by Dr. Eastman, in 

 his excellent description, to support his views, are the elongated rostrum, the prominent 

 nasal capsules and the antorbital processes, which are supposed to have served to attach 

 the pectoral tins to the head. Now, the rostrum is one of the most variable organs in 

 presence, form and size, not, only among the sharks, but also among tin; rays. In the 

 shark Centrophorm it equals in length the rest of the skull ; in the Myliobatid rays 

 it is wholly absent. As regards the antorbital cartilages of Tamiobatis, there are, it 

 seems to me, sufficient evidences that they did not support the pectoral fins. In the 

 rays these cartilages are articulated to tin; nasal capsules, and the distal extremity of 

 each is directed backward along the inner side of the fin. In Tamiobatis there is no 



* Die Eocanen Selachier Monte Bolca, 1894, p. 45. 

 \ Ze/itsehr. deuteeh. Geolog. Oeselhch., li, 1898, p. 298. 



