58 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



mations to typical rays as Taraiobatis, Archreobatis, Psammodus, Janassa, 

 etc., are genetically related to the Batoidei, as commonly recognized, may 

 perhaps be questioned, and indeed is even denied by Dr. 0. P. Hay,^ 

 since we should expect to lind greater differences than obtain between 

 modern sharks and rays, had their divergence taken place at an ex- 

 tremely remote period. There is no difficulty in supposing the dentition, 

 general configuration of the body, and most minor characters of rays to 

 have been paralleled in the Palaeozoic by adaptation to similar conditions, 

 such as bottom-living, amongst speciaHzed groups which later became 

 extinct ; and we are obliged to affirm that at present there is no evi- 

 dence to show that the essential feature of rays, namely the attachment 

 of fin-supports to the side of the head, was originated until well along in 

 the Mesozoic. 



But with respect to Campodus, we may dismiss the question of its sup- 

 posed affinities to rays on the ground that all available evidence points 

 to a close relationship to Cestracion, the arrangement of teeth being 

 essentially similar, and the mouth-cleft long and narrow, instead of wide 

 and transverse. The two rami of either jaw probably included about the 

 same angle between them as in Cestracion. Orodus and related Palaeo- 

 zoic genera undoubtedly possessed a Campodus-like dentition ; and while 

 these, together with more specialized forms such as Edestus, etc., failed 

 to survive the Palaeozoic, the primitive Cestraciont type manifested 

 great longevity. 



The lateral series of teeth belonging to Campodus are already suffi- 

 ciently well known, thus rendering further description superfluous. AVe 

 need only remark that the orientation of detached teeth may be readily 

 determined from the following characters : (1) The coronal buttresses 

 are invariably directed outward, and the longitudinal ridge on the oral 

 surface is slightly ectad of the middle line of the crown ; (2) the antero- 

 lateral and postero-lateral series increase in size on passing toward the 

 middle of each ramus, where one of the series is sensibly enlarged ; (3) 

 the coronal eminences are more elevated in one jaw, presumably the 

 lower, than in the other ; (4) interposed between the foremost of the 

 antero-lateral series on either side are the most anterior, or as we sliall 

 hereafter term them, symphysial teeth, immediately to be described. 



Symphysial dentition. — There are at present but two specimens known 

 of the symphysial dentition, both of which are illustrated in the accom- 

 panying figures, which are reproduced from photographs. That shown in 



1 Hay, 0. P., The Chronological Distribution of the Elasmobranchs. Trans. 

 Amer. Piiil. Soc, Vol. XX., p. 74, 1901. 



