104 BULLETIN OF THE 



which the nasals unite is vertically ahove, or a little in front of the mesial 

 forward bend, formed by the junction of the prenasals with the median. Con- 

 sequently the median may be described as nearly or quite vertical. The pre- 

 nasals are elongate ; they unite directly with the subrostrals, forming with them 

 a single tube on each side of the rostral cartilage, as in Myliobatis aquila. In 

 M. freminvillei, which more closely resembles Aetobatus in shape, these tubes 

 are closely applied, but remain separate. At each side of the median line the 

 oral of Aetobatus sends forward a sharp curve, and on the outside of each of 

 these a similar loop is sent outward; the tube goes some distance backward 

 from this second bend before turning outward and forward. It ends without 

 joining the angular. 



The closeness of the relationships existing between this genus and the pre- 

 ceding are asserted in the characteristics of the canal system with as great 

 emphasis as in any other portion of the anatomy. 



Rhinoptera. 



Plates 1., M. 



So far as the general features of the canal system are concerned, this genus 

 resembles both of the preceding. At the same time there are respects in which 

 it differs decidedly from either of them. The majority of these are due to 

 difference in the structure of the head, yet the divergences are not wholly con- 

 fined to this portion. Again, on comparison with Dicerobatus the indications 

 of close affinities are very conspicuous on the trunk, but on the head the rela- 

 tionship becomes apparent only on closer study, being masked by the dissimi- 

 larity in shape. 



Rhinoptera brasiliensis (Plate L.). Abrupt bends give the scapular fold in 

 this type more prominence than it would attain by a gradual curve, as it departs 

 but little from the main course of the lateral. This fold bears a pre-scapular 

 and also a post-scapular branch, and between them an elongate pre-scapular 

 and a much smaller scapular inclosure. Behind the end of the aural, on the 

 lateral, there is a strong forked occipital branch with a multitude of branchlets. 

 Leaving the scapular area the pleural goes back and outward a short distance, 

 where it has the appearance of being crowded back upon itself in a number of 

 folds; from these it extends with tolerable directness to the side of the head. 

 Its branches are few and long ; their ends are much dissected. The two tu- 

 bules in front of the posterior one are forked near the middle of their length ; 

 the hinder one branches a greater number of times. 



All of the anterior cephalic canals are affected by many flexures, as if in 

 compensation for the short distances between the extremities of the tubes. 

 Bringing the mouth so far back toward the gill-openings, and ending the snout 

 below the forehead, gives the rostral canal a vertical direction, and carries 

 orbital and pleural under the anterior part of the skull. The tubules of the 

 orbital pass forward on the inclined portion of the forehead. No distribution 

 of the canals occurs on the upper surface of the rostral fins ; the tubes seek the 



