24 SVEN LOVEN, ON POURTALESIA, A GENUS OF ECHINOIDEA. 



The spines of Pourtalesia Jeffreysi are generally slender and sparse, but stronger 

 and crowded on the sternum and more particularly on the palatal vault of the buccal 

 vecess, Pl. ],fic/. 2; IV, 24. Within the scrobicular circle of the tubercle, Pl. V,fig.31, 

 rises the perforated mamelon surrounded by a »milled ring». The perforated condyle 

 of the spine, fig. 30, is encircled by an undulated ling answering to the milled ring of 

 the tubercle, and the whole surrounded by a wide, radiately waved brim, all these 

 parts being of a dense and glossy, partly homogeneous calcareous substance. Above 

 the brim, fi.g. 32, an apparently confused mäss of compact meshy texture begins to 

 rise obconically, forining the thick basal part which ends above in »the collar». There 

 the slender calcareous fibrils of the meshes are seen, bending centrally, to unite, fig. 

 32, 33, into the regular string-like pillars that constitute the shaft, fig. 36; Pl. XIV, 

 fig. 171, joining one another all along by giving off, on either side and inwardly, 

 fig. 32, 34, 35, at regular intervals, the connecting processes that produce the well- 

 known radiated texture seen in transverse sections. This is, in general terms, the 

 structure of the spines in the Spatangids, and, in the main, of all the Echinoidea. In 

 the following it will be seen that an analogous disposition of the constituent parts is 

 met with in the rods which strengthen the filaments of the phyllodean, subanal and 

 sometimes the frontal pedicels, Pl. Vill, fig. 55, 56, 57, 60, 61, 62 and 64; Pl. XI, 

 fig. 121, 129, the basal circlet answering to »the collar», with the protruding näve 

 below, its meshes uniting above, to form the shaft which sometimes shows signs of 

 being composed of two or more string-like pillars rudely joined. 



Like the fasciolte the spines stånd in no constant and fixed relation, with regard 

 to their growth and position, to the underlying ambnlacral plates. In my former 

 work, a series of figures is consecrated to the development of the frontal ambulacrum, 

 III, of Toxopneustes droebachensis ^). In a specimen of 3 mm., the first primary 

 tubercle in III a covers the suture of the primary plates 2 and 3, in III b those of 

 plates 2, 3, 4, and so it continues in somewhat larger specimens, gradually reduced, 

 till in a specimen of 11 mm., and before that size, it aborts or is shed in III a, as 

 later in III h. And in the same specimen the primary tubercle is seen to cover 

 more or less completely the sutures of each group of plates: 4 — 6, 7 — 10, 11 — 14 (15), 

 of III a, and 5 — 7, 8 — 11, 12 — 16, of III h. In a Meoma venti"icosa ^), they are seen 

 to expand across the mesial suture of the interradia, in an Hemiaster expergitus ^) the 

 upper spines of the interradia 2 and 3 not only extend their bases partly över the terminals 

 of the ambulacra II and III, but even invade the calycinal system. Nothing, in fact, 

 is more easily observed than this independence of the radiolar system. Covered itself by 

 the fasciolar stratum, it overlies the ambulacra as well as the perisome. Its genesis, 

 the order and mode of appearance of its spines, their growth and decay, and their 

 slow change of position, are inviting subjects of research. 



The pedicellarise, which now begin to be properly studied, are in these respects 

 as little known as the spines. On those of Pourtalesia I have nothing to add to 

 the descriptions given by Wyville Thomson and Al. Agassiz. 



1) Études pl. XVII, fig. 140—147. 2) Ib., pl. XII, lig. 106. ^) Ib. pl. XI, fig. 93. 



