ASTEROIDEA 



103 



Pt. obscurus (Perr.) 

 2 

 3 



Key to the species of Pteraster known from or likely to occur 

 in British seas. 



1. Rays 6 



,,5 



2. Paxillae with 2-5 spines 



., ,, 10-25 spines ...... 5 



3. Jaws with a pair of strong spines on the outer surface 



1. Pt. militaris (O. F. Miill.) 

 Jaws without spines on the outer surface .... 4 



4. Jaws with a small median projection (Fig. 59). 5 adambulacral 



spines. Fin-like membrane inconspicuous 



2. Pt. personatus Sladen 

 Jaws without any median projection, the proximal edge straight, 

 broad, with four strong spines on each side the mid -line and 

 two smaller on the side-edge. 4 adambulacral spines. Fin- 

 like membrane conspicuous . . . Pt. sordidus Perr. 



5. Spines along edge of jaw united by a web continuous across the 



mid-line ; no median keel or anterior prolongation ; a pair of 

 large, robust spines on outer surface of jaws Pt. pulvillus M. Sars 

 Spines along edge of jaw not united across the mid -line ; jaws 

 with a median keel and a conspicuous anterior median pro- 

 longation ; spines on outer surface of jaws not very robust 

 or (?) absent . - . . . . . . .6 



6. Spines along edge of jaw equal-sized, those on each side united 



by a continuous web ; 2-3 similar spines on outer surface of 

 jaw, close behind the marginal series. 6, farther out 5, adambu- 

 lacral spines . . . . . Pt. reductus Koehler 

 Spines along edge of jaw not united by a continuous web along 

 each side, but in two groups : two larger ones on each side of 

 the median projection and four smaller ones on sides of jaw ; 

 a single small spine on outer surface of jaw, close to the two 

 larger anterior ones. 5 adambulacral spines 



Pt. alveolatus Perrier 



Fig. .59.— Jaw of Pteraster personatus. x 5. 

 (After Sladen, Echinod., S.W. Ireland.) 



