MUSEUM OF COMPARATIVE ZOOLOGY. 91 



of the main canals. If these vessels alone were taken'into consideration, more 

 than specific distinction would not be accorded the two types. 



In comparison with the preceding the laterals on the smooth Skate approach 

 each other more gradually behind the shoulders and more abruptly in front 

 of them. Over the gills the branchial area is wider, and in general it is more 

 irregular in outline. The majority of the tubules on the hinder branch of 

 the pleurals run forward or outward, and on the post-scapular, toward the 

 hinder margin, a number of them turn backward. 



The occipital i3 rather long. At the side of the eye a branch, from the 

 suborbital, turns back in the direction of the branchial area. Three or four 

 tubules connect the orbitals with the pleurals; the latter go down near the 

 margin, more than half the length of the snout in front of the skull. 



Beneath the disk the subrostral is parallel with the prenasal more than half 

 the distance to the mouth; it then turns outward, and returning makes a sharp 

 curve and fold, not qiute as close and complete as that of Uraptera, after 

 which it goes back obliquely, instead of transversely as in that genus, thus 

 bringing about a shortening of the orbito-nasal. The pleural lies close by the 

 side of the angular and orbito-nasal; opposite the mouth it bends outward 

 a short distance, then stretches back almost directly toward the posterior angle 

 of the pectoral; and about as far back as the middle of the abdomen it turns 

 to the coraco-scapular arch to meet the jugular. The space included is narrow 

 in front, and much broadened at the shoulder girdle. There is a moderate 

 amount of curvature in the nasal. The median is transverse. At the median 

 the prenasals make a broad bend ; they are not connected in front. The oral 

 is disconnected, and is in two sections. 



Plate XXVII. shows the tubes and tubules of the upper surface; Plate 

 XXVIII. gives (fig. 1) the hyaline mucous ducts of the " ampullae of Loren- 

 zini/' and (fig. 2) the main tubes of the canal system of the same surface ; 

 and Plate XXIX. contains a view of the lower side of the head in fig. 1, and 

 a sketch of the upper surface in fig. 2. 



Baia ocellata (Plate XXX.) is one of the species with shorter snouts. In 

 consequence of the rostral shortening, the lengths of the prenasals and of the 

 rostrals have been decreased so much as to bring their forward extremities 

 almost back to a transverse line from the end of one suborbital to that of the 

 other. 



Above the thoracic region the curves of the laterals are shallower, and the 

 scapular bends are less prominent, than in R. Icevis. The pleural areas are sub- 

 triangular, broader posteriorly. The posterior branch of the pleural is the 

 longer. A strong post-scapular extends from the shoulder obliquely out to 

 the posterior margin. This tube is provided with tubules on its outer half. 

 Laterals, pleurals, and orbitals also have tubules, which are more or less 

 irregular in regard to length. 



■ The occipitak are of moderate length. The cranials have a prominent 

 curve opposite the fontanelle, and another near the orbitals; their tubules are 

 short, with the exception of several in front of the orbit. 



