MARINE VERTEBRATES 



319 



the Thornback Skate (Raia davata), distinguished by the clawlike 

 spines down the middle of the back as well as on other parts of the 

 body ; and the Common Skate (R. vulgaris), a very voracious 

 species, from two to four feet long, with a very sharp muzzle. 



All the members of this family are bottom fish, without air- 

 bladders; and their eggs, which are large and detached, are 

 enclosed in horn capsules which are so commonly washed up on 

 the beach that they are well known to frequenters of the sea-side, 

 who call them Skates' Barrows or Shepherds' Purses. These cases 

 are oblong in form, with a pro- 

 cess at each corner, and the 

 material of which they are com- 

 posed looks very much like that 

 of some of the coarser sea weeds 

 after they have been dried in 

 the sun. As a rule only the 

 empty cases are cast ashore by 

 the waves, open at the end 

 where the little skate made its 

 escape ; but occasionally we meet 

 with the complete egg, and the 

 case, while still wet, is sometimes 

 sufficiently transparent to show 

 the form of the embryo within. 



Dogfishes are also fairly well 

 known to sea-side ramblers, for 

 not only are some species used 

 as food in many places, but 

 they are also frequently to be 

 seen cast aside with the refuse 

 from the fishermen's nets. The 



common Spiny Dogfish (AcantMas vulgaris}, belonging to the 

 family Splnacidce, frequents all parts of our coasts. It reaches a 

 length of three or four feet, and is of a slate -blue colour above 

 and very pale yellow below. The pectoral fins are very large, the 

 ventral fin absent, and there is a very sharp spine in front of each 

 dorsal. The creature is ovo-viviparous ; that is, the eggs are hatched 

 while still within the body of the parent. 



Another family (Scylliidce) contains two British species without 

 spines, and is also characterised by having the first dorsal fin far 

 behind. They are the Larger Spotted Dogfish (Scyllium ccmicula) 



FIG. 230.- 



-THE EGG-CASE OP 

 DOGFISH 



