PACHYCENTROTUS. 349 



The arrangement of the pore-pairs in arcs of four is a very obvious difference 

 between this genus and Loxechinus and shows its relationship to Paracentrotus. 

 The globiferous pedicellariae are unhide either of those genera but are Uke those 

 of Hehocidaris, the valves (PL 95, fig. 16) having a prominent lateral tooth on 

 the left side (PL 95, fig. 17). 



So far as known this genus is monotypic but attention may be called to the 

 fact that specimens from Albemarle Island, Galapagos, are very different in 

 color from those collected at James Island or on the mainland coast. They 

 seem to be dull red-brown, with no trace of green, so that their general appear- 

 ance is quite different from typical gibhosus. 



Pachycentrotus,^ gen. nov. 



Type-species, Sphcerechinus australioe A. Agassiz, 1872. Bull. M. C. Z., Ill, p. 55. 



Plates 94, figs. 1-6; 98, figs. 5-8. 



Although Mortensen had the opportunity to examine a specimen of this 

 species in the British Museum, taken by the "Challenger" in Bass Strait, he 

 does not attempt to "decide to which genus and species" it belongs, but simply 

 points out the most striking feature of its pedicellariae. It appears from other 

 remarks which he makes that he has seen specimens of S. granulans labelled 

 australioe and it is not strange that this has led him to doubt whether he has ever 

 seen the true australioe. There is however little doubt that the "Challenger" 

 specimen he saw in London is australioe for a similar small specimen from the 

 same station in the collection of the M. C. Z. is undoubtedly that species. 

 The globiferous pedicellariae are, as Mortensen says, like those of Heliocidaris 

 in having a large lateral tooth near the tip only on the left side but they are quite 

 characteristic nevertheless in certain details (PL 94, figs. 1, 2)\ the stalk about 

 equals the head (.90-1 mm.) and there is no neck. The tridentate vary greatly 

 in size, the valves (PL 94, figs. 3, 4) ranging from .15 to .80 mm. In the ophi- 

 cephalous, the valves (PL 94, fig. 5) are about .50 mm. long and in the triphyl- 

 lous, they (PL 94, fig. 6) measure .12 mm. The spicules in the pedicels are 

 bihamate. 



This genus is well characterized by its thick, heavily tuberculated test, with 

 ambulacra having only 4 pore-pairs (rarely 5, PL 98, figs. 7, 8) in each arc, and 

 with oculars I and V (rarely IV or II, Plate 98, fig. 6") insert. The sharp, distinct 



1 7ra;)(us = thick, stout -f nivrpov = prickle. 



