60 SVEN LOVEN, ECHINOLOGICA. 



the large external wiiig, al. exf., which makes all bnt tlie 

 whole of the inter-muscular siirface and is developed from the 

 hnmp of the pyramid, is without equivalent in those forms. In 

 Clypeaster the two wings, al. int. and al. ext. are separated 

 from one another by an interval of peculiar structure, a tri- 

 angnlar area, tr. fig. 59, the base of which coinoides with the 

 ledge of the epiphysis, while its point is directed downward 

 to the labial process. Outside, it is marked by the absence or 

 scarcity of the holes and the deviating, träns verse direction 

 of the faintly elevated lines. It indicates, from withont, the 

 space within which that part of the compact body which forms 

 the dental slide joins the inter-ranscnlar sheet, as seen in the 

 section, Pl. X, fig. 135 x X . If this sheet is carefuUy removed, 

 Fl. VIII, fig. 65, it is seen that the lameliar structnre, emi- 

 nently regular throngh almost its whole extent, is interrupted 

 along the external side of the triangle and replaced by an 

 irregnlar, invertedly dendroid structure proceeding from a 

 dense and solid part beneath the supra-alveolar process, and 

 on one side sending off lameilar branchlets, which enclose 

 shallow lacunes, towards the regular lamels of the external 

 "vving, on the other spreading theni in the internal. 



The paired anricular branches that are received eaeh in- 

 to one of the two fossee of every p^a^amid, Pl. VII, fig. 52, 

 54, 55, 56, arise from the ambulacra, and have between them 

 the very minute and narrow interradium,^ often partially or 

 wholly overgrown by their expanded base. They belong each 

 to a diiferent ambulacrum, the nearest column right and left, 

 in such a manner that, for instance, the fossa a of the pyra- 

 mid 5, fo. 5 a, fig. 58, receives its auricular branch from V h, 

 fig. 55, and the fossa b its branch from I a, and so forth, as 

 it is in the Ectobranchiates. The two of one pyramid are 

 near together, and their bases almost parallel, each branch 

 rising close to the external margin of its column. They are 

 smaller and weaker in proportion than in the regular Echi- 

 noids, compressed radially, not concentrically, and very slightly 

 bent adorally; the tumid tops lean interradially and adorally. 

 A process derived from the proper tissue of the ambulacrum 

 is the core of each branch, and is covered with a thick coat 

 of that other peculiar calcified tissue, the luxuriant peritoneal 

 system that lines with a regularly pierced crust the whole 



' Etudes, Pl. XLVII. 



