PHORMOSOMA. 153 



Actinal tube-feet in a single (or rarely two) more or less irregular series. 



Primary ambulacral plates throughout most of ambulacra accompanied, 



each by two secondary poriferous elements. 



Many actinal primary spines enclosed in skin-bags, none with 



hoofs; abactinal and actinal surfaces strikingly and abruptly 



unlike Phormosoma. 



Actinal primary spines not enclosed in skin-bags, some at least 

 ending in hoofs; abactinal and actinal surfaces not strikingly 



and abruptly unlike Echinosoma. 



Primary ambulacral plates, each accompanied by a single secondary 



element or none Kamptcsoma. 



Actinal tube-feet in three more or less distinct series. 



Each half of an ambulacrum, actinally, consists of a column of wide 

 low primary plates, each accompanied by two small secondaries. 

 Abactinal surface covered by numerous small primary tubercles 

 none of which are conspicuously larger than the others ; abac- 

 tinal primary spines encased in loose skin-sheaths Asthenosoma. 



Abactinal surface with at least 30 conspicuous primary tubercles ; 



abactinal primary spines not encased in loose skin-sheaths . . . Arceosoma. 

 Each half of an ambulacrum, actinally, consists of four columns of 

 plates, the two median columns made up of the secondary plate- 

 elements, the inner column made up of the inner halves of the pri- 

 mary plates, the outer column, of the outer halves Sperosomd. 



, Phormosoma. 



Wyville Thomson, 1872. Proc. Roy. Soc. Edinburgh, VII, 84, p. 617. 

 Type-species, Phormosoma placenta Wyville Thomson, 1, c. 



It seems to us desirable to accept Mortensen's limitation of this genus, 

 as the group is well defined and easily recognized. The test is rather rigid, 

 with actinal side markedly different from abactinal. The actinal primary 

 tubercles and areolae are large, while the loose skin-sheaths of the spines 

 are very characteristic. We beg to call attention to the arrangement of the 

 nearly spherical sphaeridia, an interesting generic character overlooked by 

 Mortensen but which seems to be very constant. They are present actin- 

 ally on both secondary elements of each primary plate. Of course one 

 occasionally finds a secondary plate-element actinally which has no sphas- 

 ridium, but it is usually clear that the absence is accidental. In all other 

 Echinothurids, only the inner (lower) secondary element carries a sphae- 

 ridium. This genus, as now limited, contains eight recognizable species, but 

 they are closely related to each other and the lines of separation are not 

 distinct. Both Mortensen (1903) and Doderlein (1906) are inclined to 

 regard rigidum A. Ag. as a synonym of placenta, in spite of its great geo- 



