The Zoologist — February, 1874. 3873 



principles on which locomotion is effected, while in others they are 

 highly valuable as affording accessory means of discriminating the 

 zoological affinities of the different races and species. 



Commencing with the Plagiostomous order, we find in the two 

 primary sub-groups, including respectively the sharks and rays, 

 that progression is effected on very distinct principles. With the 

 Selachoidea, or shark tribe, the fish move by the even, powerful 

 swaying from side to side of the largely-developed and unsym- 

 metrical caudal fin and whole posterior part of the body, the other 

 fins remaining quiescent and being merely subservient as balancers. 

 Descending to the species, we find again that each form exhibits a 

 peculiarity of action distinct from its congeners, and one which 

 readily enables us to discriminate between them. Thus in the 

 smooth hound {Mustelus) the pectoral fins are so largely developed 

 that their balancing powers are highly augmented ; comparatively 

 slow motion of the caudal extremity suffices to propel the fish 

 through the water, and the whole body being flexible, it progresses 

 with a measured grace of action surpassed by no other species of 

 its tribe. In the picked dogfish (Acanthias) the general contour of 

 the body is very similar to that of the last species, but the pectorals 

 being much smaller, more rapid action of the caudal extremity is 

 requisite for supporting it in the water, and to this has to be added 

 a great rigidity of the anterior half of the vertebral column, causing 

 the fish to swerve from side to side with each stroke of the tail, the 

 same cause preventing it also from turning corners with ease and 

 rapidity, and altogether imparling to it a want of grace of action 

 compared with that of other members of its tribe. For the fore- 

 going reason, this species requires a tank of larger size for its 

 preservation in good health than other dogfish, as if confined within 

 the boundaries of a small one, it beats its head against the sides 

 and rockwork to such an extent that the cartilage of the skull is 

 frequently exposed to view. In the spotted dogfish {Scyllium) the 

 whole body is more elastic even than in Mustelus, a character ad- 

 mirably fitting it for its ground-loving habits, and enabling it to 

 explore and adapt itself to every sinuosity of the ground while 

 hunting for its prey. When swimming in open water it is dis- 

 tinguished by a more rapid action and swifter progress than 

 Mustelus, though at the same time the greater amount of force 

 expended in its movements deprives it of the peculiar grace 

 associated with that species. 



