27G STRONGYLOCENTROTUS. 



smaller interambulacral plates, and in older specimens the plates become 

 deeply embedded in the buccal membrane. The pores at first are placed 

 in a vertical row in very young specimens; the}' then become arranged 

 in arcs of three or four pairs : with increasing age the median rows of in- 

 terambulacral tubercles assume the arrangement found in the adult. Owing 

 to the rapid growth of the spines in the young, the extremity, and fre- 

 quently the greater part of the spine almost to the base, is hollow; but 

 as the young increase in age they become more solid at the base, and 

 further up in proportion to their age. The anal tube of small specimens 

 is very striking, being as prominent as the anal proboscis of .-oine species 

 of Comatulae (PL IP. f. u). The anal membrane is still quite flexible in 

 the adult, and forms a proboscis, though shorter proportionally than in the 

 young, as Grube pointed out some time since. 



In very young Echinothrix turcarum the spines become early solid, and 

 differ generally strikingly, on account of the close verticillation forming a 

 longitudinal striatum, from those of Diadema or of the other species of 

 Echinothrix. I was inclined formerly to retain for this species the generic 

 name proposed fust by Gray, but having examined a large series of this 

 species in the Godeffroy Museum, I find that although the spines are more 

 closely verticillate, yet the solidity — the main character upon which the 

 generic separation was based — was not so universal as I had been led to 

 suppose, and the structure of the test presenting no feature by which the 

 species of this group could be distinguished generally, they have been 

 united with Echinothrix. 



ECIIIXOMETRADAK. 



Family Echinometradae Gbay, 1855. Proc. ZooL Soc. London, p. 37. 



STRONGYLOCENTROTUS. 



Strongylocentrotus BbanDT, 1835. Prod. Descript. An. 



This genus was first established by Brandt for the typical Echinus 

 chlorocentrotus = Drobachiensis. It has, in the subsequent limitations of the 

 genera into which Echinus has been divided by Agassiz, Desor, A. Agassiz, 

 Liitken, Verrill, completely escaped their attention. It includes all species 

 having a somewhat circular or subpentagonal, regularly arched or slightly 



