38-4 THE VOYAGE OF H.M.S. CHALLENGER. 



of irregular form are imbedded. The oral and excretory openings are on the upper 

 surface, a little behind the anterior border of the ambulatory tract, and a little in advance 

 of the posterior extremity of the body respectively. A slightly elevated pyramid of five 

 very accurately fitting calcareous valves closes over the oral aperture and the ring of oral 

 tentacles, and a less regular valvular arrangement covers the vent. 



"In the middle of the back in the female there is a well-defined saddle-like elevation 

 formed of large tessellated plates somewhat irregular in form, with the surfaces smoothly 

 granulated (fig. 139). On removing one or two of the central plates we find that they are 

 not, like the other plates of the perisome, imbedded partially or almost completely in the 

 skin, but that they are raised up on a central column like a mushroom or a card-table, 

 expanding above to the form of the exposed portion of the plate, contracting to a stem or 

 neck, and then expanding again into an irregular foot, which is imbedded in the soft 

 tissue of the perisome ; the consequence of this arrangement is that when the plates are 

 fitted together edge to edge, cloister-like spaces are left between their supporting columns. 

 In these spaces the eggs are hatched, and the eggs or the young in their early stages are 

 exposed by removing the plates (fig. 140). At first, when there are only morales or very 

 young embryos in the crypts, the marsupium is barely raised above the general surface 

 of the perisome, and the plates of the marsupium fit accurately to one another (fig. 139) ; 

 but as the embryos increase in size, the marsupium projects more and more, and at length 

 the joints between the plates begin to open, and finally they open sufficiently to allow the 

 escape of the young. The young in one marsupium seem to be all nearly of an age. 

 In Psoitis ephippifer the marsupium occupies the greater part of the dorsal surface, and 

 its passages run close up to the edge of the mouth, so that the eggs pass into them at 

 once from the ovarial opening without exposure. 



" In the male there is, of course, no regular marsupium ; but the plates are arranged 

 in the middle of the back somewhat as they are in the female, except that they are not 

 raised upon peduncles ; so that it is not easy at once to distinguish a male from an 

 infecund female. 



"Although we have taken species of Psolus sometimes in great abundance in various 

 parts of the world, particularly in high latitudes, southern and northern, I have never 

 observed this peculiar modification of the reproductive process except on this one 

 occasion. 



" On the 28th of January 1876 we dredged from the steam pinnace in about 10 fathoms 

 water off Cape Pembroke, at the entrance of Stanley Harbour, Falkland Islands, a number 

 of specimens of a pretty little regular sea-urchin, Goniocidaris cancdiculata, A. Agassiz. 



" The genus Goniocidaris (Desor) seems to differ from the genus Cidaris in little else 

 than in having a very marked, naked, zigzag, vertical groove between the two rows of 

 plates of each interambulacral area, and one somewhat less distinct between the ranges of 

 ambulacra! plate3. It includes about half a dozen species, which appear to be mainly 



