MUSEUM OF COMPAEATIVE ZOOLOGY. 71 



near the median. Aetobatus agrees with M. aquila in regard to junc- 

 tion of subrostral and prenasal ; it differs from both of the sketched 

 species of Myliobatis in such characters as would be more likely to be 

 induced by greater compression of the head, as is seen in the foldings 

 of the oral, deeper curvature of the aural, etc. 



Zygobatidse. — In this family the mouth, as compared with that of 

 the Myliobatidse, has been drawn backward closer to the gill openings 

 or the coraco-scapular, and the snout, through reduction, and retraction 

 to follow the mouth, has arrived at a position beneath the skull. The 

 process of the change is well written in the foldings, sinuosities, and 

 contortions of the cephalic tubes. Besides these particulars of charac- 

 terization there are others, apparently resultant from a shortening of 

 the longitudinal axis of the body, equally peculiar in this group. Of 

 these are the scapular and posterior pleural foldings. The species 

 figured differ in the number of cephalic tubules, in the scapular areas, 

 in the post-pleural folds, and in the oral, which is divided in one 

 species, united in the other. 



Between the Dicerobatidse and the Zygobatidse there are more 

 points of resemblance than between the former and the Myliobatidae. 

 There is more of a disposition to mass the pleural tubules posteriorly 

 than in either ; and the connection of the laterals across the vertebrae 

 has not before been noticed in any of the Batoids. A further peculi- 

 arity occurs on the inner anterior section of the lower pleural, on which 

 the tubules turn backward, as in Myliobatis freminvillei. The latter 

 is no doubt only a specific character. Of the cephalic canals, it is 

 hardly necessary to say anything, the distinctions arising from the 

 peculiar shape of the head are so excessively marked. Yet, as is 

 noted in the description of D. olfersii, the affinities existing between 

 Dicerobatus and Ehinoptera are shown by the canals of both body 

 and head. 



The subjoined synoDtic list furnishes a condensed illustration of the 

 availability of the canal system in classification. Being based exclu- 

 sively on the species here described and figured, some of them repre- 

 sented by single specimens, it is to be expected that study of new forms 

 will necessitate modification and rearrangement. 



