STRONGYLOCENTROTUS DROBACHIENSIS. 277 



depressed test, with smooth, imperforate, not crenulate tubercles of unequal 

 sizes, forming primary and secondary vertical rows. Pores arranged in arcs 

 of at least four to five pairs. Actinostome decagonal; very slight cuts; buccal 

 membrane bare ; spines moderately slender, longitudinally striated, longer 

 proportionally than those of true Echinus, and more slender than those of 

 Sphaerechinus. 



The analysis of the mode of growth of the pores shows how impossible it 

 is, upon the mere question of quantity or direction of the pores, to subdivide 

 this genus as has frequently been attempted ; and now that it is associated 

 with the Echinoinetradae, instead of the Echinidae, where we find variations 

 similar to those noticed in Strongylocentrotus, the homogeneous nature of the 

 genus as now limited cannot fail to be apparent, and however convenient the 

 former generic sections may be as keys for ready identification of species, we 

 cannot allow them greater value. The modifications of the poriferous zone 

 upon which these so-called generic distinctions have been based are in this 

 case simply quantitative modifications, not involving a single structural fea- 

 ture, as an examination of the analysis of the plates of the poriferous zones 

 of Echini apparently shows ; the petaloid appearance of the zones imme- 

 diately round the actinostome is a mere result of the flattening of the test, 

 and the direction of the arcs of pores due to the greater or less rapid growth 

 of the new additional plates of the poriferous zones or of the primitive plate 

 of each arc. If the latter grows rapidly, the arcs are open and transverse 

 (Loxechinus) ; if it is the older plates which grow very rapidly, we have very 

 regularly arched arcs, each arc (Toxocidaris) of greater or smaller size if 

 the new plates are added only at the upper part of the arc ; while if they 

 are also added at the open end of the arc, the arcs are more vertical 

 (Sphaerechinus), a structural feature which, taken into connection with 

 the actinal cuts, seems to entitle the section (Sphaerechinus) to rank as 

 subijeneric. 



o 



Strongylocentrotus Drbbachiensis 



! ? Echinus Drbbachiensis Mull. 1776. Zool. Dan. Prod. 



! Strongylocentrotus Drobachiensis A. Ag 1872. Rev. Ech. Pt. 1. 



PL IV". f. 2, 3, 4 ; PI. IX; PL X. 



After a careful re-examination of the whole subject I am unable to distin- 

 guish more than one species of Strongylocentrotus in the circuinpolar speci- 

 mens thus far collected. I am aware that there are marked local differences, 



