78 BULLETIN OF THE 



of the subrostral on the upper surface to retain its position outside of the nos- 

 tril, the latter being on the edge. And the orbitals, being behind the expan- 

 sion, would sink deeper into the tissues; this also is simply going farther in 

 the direction already partially traversed by the orbitals of Scoliodon. 



Mtistelus. 



Anteriorly on Mustelus canis (Plate VIII.) there is hardly any departure 

 from a right line in the lateral. Over the anal fin the canal rises ; farther back, 

 it descends to its former level, and, not going below the middle of the caudal 

 muscles, it stops at the last of the vertebrae. 



Comparatively little curvature is apparent in either aural or occipitals. 

 The latter are short and diverge toward the cranials. In front the cranials 

 converge ; opposite the fontanelle they turn directly toward the side of the 

 head for a short distance, then they run forward, almost straight, somewhat 

 convergent, and pass through the snout before reaching the tip. At first the 

 orbitals are transverse, but with a gradual curve they sweep below and about 

 half its diameter in front of the eye, where they turn back and downward to 

 meet the angular beneath the anterior third of the orbit. The angular is of 

 medium length ; the jugular is short, ending near midway from spiracle to gill 

 opening. A short horizontal orbito-nasal connects with a long subrostral, in 

 which there is but a slight bend at the side of the nostril. A prominent curve 

 appears in each nasal, between the nostrils. The median is long and longi- 

 tudinal. The prenasals are rather long; they connect with the rostrals. 

 Behind each ang;e of the mouth there is a short detached oral. 



The canal system of Mustelus closely resembles that of Triacis. 



Triads. 



On Triacis semifasciatum (Plate IX.) there is hardly any curve in the scapu- 

 lar portions of the laterals. As in Mustelus, the anal cui've is a broad one; 

 the canal does not descend to the edge of the fin, and it stops at the end of the 

 column. 



There is a slight forward bend in the middle of the aural, otherwise it is 

 almost straight. The occipitals are of medium length, and are divergent. In 

 their coronal portions the cranials are nearly straight. At the sides of the fon- 

 tanelle the bend is abrupt, but not deep. The rostral sections of these canals 

 vary in outline, converge, and descend before reaching the end of the rostrum. 

 Passing outward, the orbitals bend back slightly; they sink deeply into the tis- 

 sues behind the eye, and extend in front of the orbit more than its diameter. 

 A much more open loop is made by these tubes as they turn to join the angu- 

 lar than in Mustelus. The angular is of moderate length; the jugular is short 

 and turned up toward the superior edge of the gill opening, as in Ginglymo- 

 stoma, Scylliorhinus, and Mustelus. The orbito-nasals are short, convergent 



