270 EC^HTNODERMS OF THE BRITISH ISLES 



skeleton, while in other Echinoids the poison gland hangs 

 free on the outer side of the valve, covered only by the 

 epidermis. 



The rather numerous recent forms of this order are all regarded 

 as belonging to only one family, while some of the, likewise very 

 numerous, fossil forms must form separate families. 



I. Family Cidarid^ 



Same characters as those of the order. From the other 

 families known, the palaeozoic Archaeocidaridae and the cretaceous 

 Tetracidaridse, this family differs in having only two series of 

 coronal plates in each interambulacrum. 



Two genera are known from the British seas, but one more, 

 Stereocidaris Pomel, is almost sure to be represented there also, 

 viz. the species Stereoc. ingolfiana Mrtsn., which is widely dis- 

 tributed over the Northern Atlantic, known from off the 

 Faroes and S. of Iceland, from off Portugal and the Cape 

 Verdes, and, further, from the West Indies and off Florida ; 

 bathymetrical distribution ca. 300-1745 m. One more genus, 

 Stylocidaris Mrtsn., may perhaps also be found there, the species 

 Styloc. affinis (Philippi) (Syn. Cidaris affinis Philippi), which 

 was originally known only from the Mediterranean, having 

 been found in the East Atlantic from 33° N. down to the Cape 

 Verdes ; also known from the West Indies and off Florida. 

 Both these species have formerly been confounded with Cidaris 

 cidaris. 



Key to the genera of Cidaridce Tcnown from or likely to be 

 represented in the British seas. 



1. Surface of primary spines very finely striated, without " hairs " ; 



collar ca. 5 mm. long, often sw^ollen ; only one sort of pedi- 

 cellarise, fiat, with only two valves 2. Poriocidaris Mrtsn. 



Surface of primary spines with fine hairs, not finely striated ; 

 collar only ca. 1 mm. long ; three kinds of pedicellariae, large 

 and small globiferous, and trident ate .... 2 



2. Large globiferous pedicellarise with a distinct end tooth 



1. Cidaris Leske 

 Large globiferous pedicellari?o without a distinct end tooth . 3 



3. Primary spines usually winged ; test very closely covered with 



small tubercles ; large globiferous pedicellarise with a large 

 elongate opening (Fig. 150, 4), stalk simple ; small globiferous 

 pedicellariie without entl tooth. (Cf. ''Jngolf" Ech., i. p. 38, 

 PI. VI. 1-4, 11 ; PI. XVI. 1) . . . Stereocidaris Pomel 



