288 ECHINODERMS OF THE BRITISH ISLES 



life a beautiful violet or purple, the colour not keeping very well 

 in alcohol. Grows to a size of at least 220 mm. horizontal 

 diameter. 



In British seas this species has been found in the Faroe 

 Channel, warm area (59° 28' N., 8° 1' W., 1100-1300 m., " Michael 

 Sars "), and off S.W. Ireland (51° 15' N., 11° 47' W., ca. 1275 m., 

 " Helga "). It is elsewhere known from south of Iceland to the 

 Azores and Cape Verde, and is also recorded from the West 

 Indies. Bathymetrical distribution ca. 300-2300 m. 



[II. Suborder. Stirodonta Jackson.] 



Test usually very compact, never flexible. Ambulacral plates 

 simple or, when compound, of the Diadematoid type (Fig. 155, 1), 

 or only consisting of two primary plates. Primary spines usually 

 strong, not hollow, with an outer layer, which may, however, be 

 reduced to a small cap, found only on the oral primaries. 

 Tubercles imperforate (in the recent forms), crenulate or 

 smooth. Teeth with a keel on the inside ; epiphyses of the tooth 

 pyramids not joining above the teeth (Fig. 146, 2). Only 

 tridentate and ophicephalous pedicellarise present. 



No family of this suborder is known from the British seas, 

 but one of them, the Saleniidse, may be expected almost with 

 certainty to occur there, and another, the Arbaciidae, may 

 perhaps also prove to be represented there. 



Key to the families of the Stirodonta likely to he represented 

 in British seas. 



Primary spines long and slender, usually thorny, with a well -developed 

 outer layer ; only one on each interambulacral plate. Secondary 

 spines flat, appressed. Tubercles crenulate. Ambulacral plates 

 simple or only composed of two primary plates ; pores in a 

 single series. A large, angular plate, the "suranal" plate, on 

 the periproct ..... Fam. Saleniidce 



Primary spines usually short and stout, smooth, exceptionally 

 {Coelopleurus) long, slender, curved. Tubercles non-crenulate. 

 Outer layer of primary spines usually (excepting Coelopleurus) 

 reduced to a small cap on the oral primaries ; one or more 

 primary spines (tubercles) on each interambulacral plate. 

 Secondary spines not flat, appressed. Ambulacral plates com- 

 posed of three, rarely more, primary plates ; pores in three 

 series. Periproct covered by 4, rarely 5, symmetric valves 



Fam Arbaciidce. 



