VERTEBRATA. 253 



better-developed brains than any other fishes. Dog- 

 fishes are common, and may easily be obtained for 

 study. The cartilaginous skull is often found on the 

 sea-shore at certain seasons — an oblong object which 

 is very puzzling at first sight. The egg-shells also 

 are often seen : those of the dog-fish are transparent 



•Egg of a Dog-fish, Scj/IKum. 



and greenish, with four long tendril-like strings, by 

 which they attach themselves to seaweeds : those of 

 the skate have four short horns instead, and have a 

 black shiny surface which is suggestive of a beetle. 

 The skin of the Dog-fish is rough with small bony 

 granules, which are ossifications of the projections or 

 papillae of the dermal layer of the skin. These were 



