BIHANG TILL K. SV. VET.-AKAU. HANDL. BAND 13, AFD. lY. N:0 10. 7 



to form a single compound plate bearing three pores; in tlie 

 recent Echinoconid the peristomal plates of tlie ambulacra are 

 simple, very large, and have each of them only a single pore. 

 In the same mauner the interradia of the recent form enter 

 into the peristome with very large single plates, while in the 

 Pygaster semisulcatus each interradium begins in the peristome 

 with a single, minute, median plate, Pl. 2^ jig. 12 ^). 



These are great differences in the external features. In- 

 ternally the disposition of the peristomal parts accords better 

 The pg. 12, 13, PL 2, represent these parts in the Pygaster 

 semisulcatus after the filling mäss has been removed. It is 

 then seen that in every ambulacrum, while the poriferous 

 zones converge, the live or six first, short, peristomal plates 

 expand laterally and at the same time become gradually height- 

 ened so as to form by their succession two ridges, rising ado- 

 rally and terminatiug each with a slightly concave surface 

 rather steeply slopiug towards the peristome. The two ridges 

 are not parallel, but slightly divergent aborally. This struc- 

 ture is comparable to the auricles of the recent form, Jig. 2, 5, 

 in every ambulacrum a pair of them, the upper surface of wdiich, 

 rising aborally, answers to the surface sloping adorally in the 

 Pygaster. I think I find a structure like this to subsist also 

 in Holectypus, and to be clearly indicated in the impression 

 of a peristome described and figured by Cotteau -). But most 

 distinctly it is shown by the description and figure given by 

 this author of a flint mould of the Pilens hemispliKricus De- 

 SOR •^). Near the peristome there are in each ambulacrum 

 »two impressions, long, narrow and very deep, left by the 

 strongly developed auricles that supported the dental appa- 

 ratus». Nothing can be more nearly similar to the structures 

 described above. The ambulacra present a contracted middle 

 part, with the zones converging, and expand laterally, thus en- 

 closing the empty hollows on the walls of which the impressions 

 of their plates are seen to descend continuously, representing 

 by their succession the auricular ridges that once filled them. 

 They diverge aborally. Though evidcntly belonging to the 

 ambulacra, owing to the expansion of these they had the ap- 



') Compare Holectypus depressus Gott., in my Etudes s. 1. Ech., Pl. 

 XIV, fig. 124. 



^) Holectypus drogiacus CoTTEAU, Pal. Franc, Jura, p. 431, 432, pl. 

 108, f. 6, 109, f. 2. 



3) Ibidem, p. 448, pl. 115. 



