444 STEONGYLOCENTROTUS GIBBOSUS. 



want of sufficient material prevents me from having a positive view on the 

 question. The number of primary tubercles is also greater ; in Brazilian 

 specimens there are fifteen coronal plates, while specimens of S. lividus have 

 only twelve, — a result which may be due to the greater flattening of the test ; 

 the corresponding tubercles of the ambulacra] and interamhulacral tubercles 

 are smaller, though their arrangement, as well as that of the miliaries on the 

 coronal plates, is identical in the two species. The plates of buccal mem- 

 brane are also correspondingly larger in S. lividus ; no difference could be 

 noticed in the pedicellariae of the buccal membrane or test, as far as ex- 

 amined in one of the specimens. The accompanying comparative measure- 

 ment of S. lividus and S. Gaimardi show the points of difference. 



Brazil. 



Strongylocentrotus gibbosus 



! Echinus gibbosus Vu... 1847, in Ac. Dks., C. R. Ann. Sc. Xat. VTI. 

 ! Strongylocentrotus gibbosus A. Ai;.. 1872, Rev. Ech., Pi I. p. 164. 



Test slightly depressed ; abactinal system sunken ; abactinal region some- 

 what asymmetrical ; the upper part of test being frequently distorted and 

 enlarged by a parasitic crustacean * which forces its entrance into the anal 

 system and soon attains its full size, so that small specimens of this sea-urchin 

 are comparatively quite distorted ; •• it causes a dilatation and malformation 

 of the intestine which eventually forms a huge membranous cyst or sac, 

 often in the larger specimens extending from the summit to the lower side 

 of the shell along one side, to which it is attached by fibrous tissues. A 

 large opening is always maintained externally, out of which the claws of the 

 crab may be thrust ; but it is apparently not large enough to allow it to go 

 entirely out when fully grown." 



The outline of the test is somewhat pentagonal ; test comparatively thin. 

 The poriferous zones are broad, each nearly as broad as the median ainbu- 

 lacral space, which carries two principal outer rows of primary tubercles and 

 smaller inner rows, forming in small specimens only an irregular median 

 vertical row. In tin 1 interambulacra there are two principal vertical rows 



* Fabia Chilensis Dana (Verrill, Notes on Radiata, p. 306.) 



