266 ARBACIA PUNCTULATA. 



of the poriferous zone in older stages, they seem to belong to the Poly- 

 poridae. 



The changes which the tubercles undergo in their growth are very 

 marked; it constantly happens that primary tubercles, in way of formation. 

 become resorbed by the rapid growth of the test; at least, I can account 

 for the variation of the abactinal part of the interambulacral areas in no 

 other way (compare/. l' t , 10, IS, PL V). Young specimens of such allied 

 species as A. punctulata and A. pustulosa can readily be recognized by the 

 differences of the abactinal system, which is quite pentagonal in young spe- 

 cimens of A. pustulosa (PL V. f. ID, ISO), while the position of the ocular 

 plates relatively to the genital plates presents in this system a totally differ- 

 ent appearance from A. punctulata (PL V. f. 14, is), where the ocular plates 

 project beyond the line of the genital plates. 



The variations which occur at different stages of growth of this species 

 show how difficult it must be to distinguish the various species which have 

 been described at different times, many of which undoubtedly rest upon no 

 more substantial basis than variations due to peculiarities of young stages 

 which have been more or less persistent. The adult (when examining a 

 large series) shows no less liability to variation in the height of the coronal 

 plates, the size of the tubercles, their number upon the sides of the test, and 

 the relative number of the larger and smaller tubercles, as well as the spines 

 they carry; the sculpture even of the plates of the abactinal system, and 

 their shape and size, being subject to considerable variation. 



The specimens collected by Mr. Pourtales seem to show conclusively that 

 the species distinguished as E. Davisii in the second number of the Museum 

 Bulletin is only a local variety. Allied species of Arbaciadae are diffi- 

 cult to distinguish; and the characters by which E. Davisii was separated 

 from A. punctulata are found, in the large series of young specimens col- 

 lected by Mr. Pourtales at Cape Fear and Florida Keys, to have no perma- 

 nence. Lutken considers the Echinus pustulosus Lam. as a nominal species; 

 quite a number of specimens of it were brought home by the Thayer 

 Expedition from Brazil ; the large series we possess proves its identity with 

 A. aequituberculata. 



Littoral, — 125 fathoms. 



