26 



S. LOVEN, ON POURTÅLESIA, A GENUS OF ECHINOIDEA. 



and aproctic, FL XIV, fig. 163 — 171. The stoma, inside the general envelope, is covered 

 över by the pentagonal buccal niembrane, fig. 165, in which a few crooked and branching 

 spicules indicate the future calcilied laminte, and this meinbrane, like the whole of the enve- 

 lope, is entire, the Oäsophageal end of the alimentary canal, still closed, touching it on the 

 inside, but not piercing it, fig. 167. This, however, is' soon to be done. Another young spe- 

 cimen of the same species, which I owe to the kindness of Mr John Murray of the Challen- 



ger Expedition, a little larger than that 

 just described and slightly more advan- 

 ced, — the phyllodean pedicels just 

 begin to develop their filaraents, and 

 the spines to bend and faintly to indi- 

 cate the partings so characteristic of the 

 adult, — has the membrane of the stoma 

 pierced in its very centre by the oeso- 

 phageal opening just formed, the pressure 

 from "within causing it to protrude a 

 little, while the calcareous spicules, some- 

 what more numerous, tend to collect in 

 a circle all around. In both of these 

 young specimens of Abatus the peristome 

 is already constituted by five pairs of 

 ambulacral plates and five single inter- 

 i-adials. 



The same is seen in the j^oung of 

 Echinocardium flavescens O. F. M., 1,7 

 mm. in length, having just acquii'ed the 

 final shape, Pl. XV, fig. 172 — 183. Altho,ugh smaller than the young of Abatus caver- 

 nosus, it is more developed, the alimentary canal being opened at both its extremities. 

 The test presents a pentagonal outline. The stoma is placed somewhat before the 

 middle, fig. 172, at the two fifths of the entire length, the periproct, fig. 173, at the 

 three fifths of the same. The pentagonal peristome ^) is composed of the five still 

 narrow ambulacra, forming its angles, and of the five broader interradia, constituting 

 its sides. The meshy tissue of the test is still rather transparent, and that of the. 

 ambulacral system more dense and compact, its meshes being markedly smaller and 

 less open than those of the interradial system. It raay also be seen, that each plate 

 of the first pair of ambulacrals, close within the peristome and on its internal surface, 

 sends out from its lateral margin on either side, across the adjoining interradium, a 

 somewhat tapering lamina of compact texture. Viewed from the outside by transpa- 

 rency, fig. 174., these laminaj seem to form five narrow uninterrupted ridges running 

 across the interradial areas parallel to the peristome, but seen from the inner side. 

 Hg. 183, they are löst midways under a thick mäss of calcareous meshes. These streng- 



Periätome of a young specimen of Abatus cavernosus Phil. 



') Compare Etiules p. 14, pl. III, fig. 32, 33—35; pl. V, fig. 46, 47. 



