SPEROSOMA. 195 



trace (as Mortensen ("Ingolf" Ech. II, p. 171, fig. 27) has done in a very 

 young specimen) the development of the ambulacral plates from their 

 origin next the ocular plate, where we find a primary with a secondary 

 element above and another below it, the three of nearly equal size ; through 

 the stage where the primary is much the largest and much wider than high, 

 while the upper secondary has taken an outer, the lower an inner position ; 

 through another stage where the secondary elements have so encroached 

 on the primary that the outer and inner halves are only narrowly connected, 

 the pore-pair being in the outer half ; to the full four-column arrangement 

 of the actinal side, where the plates are more or less nearly of a size 

 and the distinction between primary and secondary elements is almost 

 obliterated. 



Mortensen (1903, "Ingolf" Ech., I, p. 63) says of Sperosoma that 

 sucking-discs are well developed on the feet. We judge this is a slip of 

 the pen, for observation on many specimens indicates that they are wanting, 

 or rudimentary, as shown in his PI. XIV, fig. 4- 



There seem to be six recognizable species in this genus, which may be 

 distinguished as follows : 



'o l 



Abactinal ambulacral plates not twice as numerous as actinal. 



Primary spines of abactinal surface numerous, 150-500 or more in 

 specimens over 100 mm. h. d. 



Abactinal tube-feet few in an imperfect double series ; poriferous 

 zone very narrow ; actinal primary spines rather less than 

 150 Grimaldii. 



Abactinal tube-feet more numerous in three unequal series, a 

 more or less distinct quincunx arrangement being evident; 

 poriferous zones often quite broad; actinal primary spines 



rather more than 150 quincunciale. 



Primary spines of abactinal surface few, never exceeding 125 and 

 usually fewer than 75, sometimes nearly or quite wanting. 



Abactinal primary tubercles very small or wanting; ambulacra 



much wider than interambulacra giganteum. 



Abactinal primary tubercles large, with areolae often occupying 

 whole height of plate ; ambulacra about as wide as inter- 

 ambulacra obscurum. 



Abactinal ambulacral plates more than twice as numerous as actinal. 



Abactinal tube-feet in two distinct series ; no ambulacral plates 



extending across an entire half-ambulacrum biseriatum. 



Abactinal tube-feet in a single crowded series ; some ambulacral 



plates extend across the entire half-ambulacrum durum. 



