70 BULLETIN OF THE 



Dasybatus has a considerable part of the anterior portion of the lower 

 pleural close to and parallel with the anterior edge of the pectoral. 

 Along this section the tubules are numerous. Backward they are dis- 

 tributed sparingly, if present. The features possessed in common by 

 the various species are best seen on the dorsal surface, since the ten- 

 dency toward variation has been much more active beneath the disk. 

 The suborbital alone is sufficiently different to distinguish the three 

 species figured : in D. nudus it is excessively elongate, it encloses a 

 peculiar series of spaces, and twice, in a couple of long reaches, it 

 comes abruptly to an end ; in D. dipterurus it is rather short and 

 somewhat sinuous ; and in D. tuberculatum it is greatly lengthened by a 

 complex series of zigzag folds or convolutions. 



Pteroplatea differs greatly from the other genera in the distribution 

 of tubes and tubules on the dorsal surface. They are arranged to 

 reach the margins around the entire pectoral, and, though numerous 

 posteriorly, the branchlets are matted in multitudes in front. On 

 P. valenciennii the tubules are most plentiful, on P. marmorata less 

 abundant, and on P. hirundo still more scattered. These species dif- 

 fer in the branchings and areas on the scapular region, as also in the 

 general arrangement and abundance. 



The Myliobatidae, through narrowing the pectorals at the side of the 

 head, have had pleurals, orbitals, and rostrals brought close together 

 under the orbit. Those types which have the fins most reduced, or 

 absent, in this location, have these tubes arranged almost vertically one 

 above another on the side of the face. This family and the Zygobati- 

 dse agree in this respect ; they also agree in restricting the dorsal 

 canals to about half the distance from the vertebrae to the outer angle 

 of the pectoral, in the arrangement of the lower pleurals in pairs of 

 lines along the anterior margins or along the branchial and the abdomi- 

 nal areas, and in having a large portion of the oral longitudinally ex- 

 tended, as if folded with compression of the head, among the more 

 noticeable peculiarities. We might also have included the Dicerobatidae 

 in the majority, if not in all, of these agreements. These families are 

 readily separated by means of the cephalic canals. Forward from the 

 fontanelle in the Myliobatidae the orbitals cross the rostrals, a position 

 they do not attain in any of the lower families. Myliobatis freminvillei 

 has longer canals on the rostrum and a less number of pleural tubules 

 than M. aquila, and it has the subrostral separate from the prenasal. 

 M. aquila has the shorter anterior cephalic tubes, the greater number 

 of tubules on either surface, and has the subrostral joining the prenasal 



