MUSEUM OF COMPARATIVE ZOOLOGY. 77 



diuni; the orbitonasal is short; the nasal is transverse, with a broad curve; 

 the median is very short; and the prenasals are long and connected with the 

 rostrals. There is a short disconnected oral behind each side of the mouth. 

 In their principal features, the canals represented in this form are interme- 

 diate between those of Mustelus and Scoliodon. 



Cestracion (Kleix). 



Forward, the laterals of Cestracion tiburo (Plate VII.) are nearly straight; 

 backward, they have a pronounced curve, between anal and second dorsal, but 

 do not descend much below the middle of the tail, and they stop near the end 

 of the vertebral column. 



The flatness of the head, the expansion of the snout, and the positions of 

 nostrils and eyes at such great distances from the occiput, have caused some 

 very peculiar contortions of the cephalic tubes. A small amount of backward 

 convexity is presented by the aural. The occipital is elongate and turned 

 toward the side. Between the eyes the cranial is very tortuous, turning upon 

 itself several times before taking its way toward the rostrum, where it again 

 makes a backward run before passing through. From the occipital the orbital 

 goes outward and backward a short distance, then passes through, behind the 

 lateral cartilages of the skull, to the ventral surface. Below the head the sub- 

 rostrals tend laterally near the edges until about half-way from the snout to 

 the eyes, where they ascend and run for a similar distance on the upper surface 

 before descending again just in front of the eyes, thus passing around the nos- 

 tril, and finally going with much directness to meet the nasal about half-way 

 from the eye to the median. A great bend out in the direction of the eye is 

 made by the suborbital on the way forward to its junction with the angular, 

 the entire length of the tube being about three times the distance between the 

 point of appearance on this surface and that of the meeting. The orbito-nasal 

 is very short; it lies at right angles with the angular. Angular and jugular 

 are about equal in length. Xear the corner of the mouth the oral is bent 

 toward the thorax; it is connected with the angular, but does not cross the 

 symphysis. Were it not for a long narrow loop putting out toward the nostril 

 the nasal would be described as nearly transverse. This loop has parallel 

 sides, is slightly bent back, and has a tubule from its extremity. A median 01 

 moderate leugth gives rise to a pair of prenasals, which are somewhat curved, 

 and which meet the subrostrals a little toward the side from the points at 

 which the latter make their appearance. 



An arrangement of canals such as that here described might be developed 

 from a form like Scoliodon terrce novce by crowding the snout back toward the 

 skull and expanding the head at the sides. To push the long cranials back 

 toward the occiput would bring about the identical curves appearing on the 

 Hammerheads ; in fact, the curves on Scoliodon are just what would naturally 

 lead to such a result. Expanding the head would necessitate the appearance 



