,30 SVEN LOVEN, ECHINOLOGICA. 



taclied biiccal sets. As long as contaiiied in the fixed part of 

 the corona the ambulacrum is as different as possible also from 

 that of the Cidaridie, charaeterized by their narroAv series of 

 similar simple, entire, primary plates. But as soon as set free 

 in the bnecal membrane, the ambulacrum of Asthenosoma as- 

 sumes, in its own way, the Cidaridean mode of conformation 

 and has its plates one by one remodelled after a similar type, 

 every plate retaining the pore and the j)edicel. 



The plates of the iuterradia are not greatly different in 

 outward form from those of tlie anibulacra. They are nume- 

 rous, low and broad, each of their columns at the ambitns 

 nearly e(jualling in breadth the two of the anibulacra. On the 

 dorsal side they feebly bend adorally in the middle. But they 

 imbricate throughout in the opposite direction, aborally, towards 

 the calyx, /?//. 161. and emit downward, adorally. at the mesial 

 siiture, an internal thin lameilar lobe projecting under the 

 preceding plate. Laterally they overlap the ambulacra. On 

 the dorsal side they are lower, feebly contracted at the middle, 

 and imbricate near the mesial suture as well as laterally. They 

 are totally excluded from the buccal membrane, being abraded 

 adorally. and in the narrow naked interstiee that thus arises 

 between their margins and the aboral borders of every two 

 ambulacral areas, are seen, besides the gills, a few minute ir- 

 regular spicules. 



A .structure like this of the ambulacral and interradial 

 systems in the genera Asthenosoma and Phormosoma is not 

 one of late origin. It existed in the Echinothuriee of the Chalk 

 and was fully developetl in the genus Pelanechinus of the 

 Oolite carefully studied by KEEPiNrr' and Groom.- There is the 

 same disposition of the ambulacra which at the ambitns are 

 fully half as Ijroad as the interradium and gradually contract 

 towards the peristome, each compound plate consisting of one 

 aboral entire plate and two adoral minute demi-plates, which 

 three together externally give rise to ascending rows adorally 

 begun by the pore of the entire plate. The imbricated scales 

 of the buccal membrane are strikingly similar; they are all 

 ambulacral, to the exclusion of the interradials. The spinary 

 system also is the same, and there can be no doubt that this 

 type was developed already in the secondary era. And much 



' Quarterlv .Tournal Geol. Soeiety. n:o 13tj. XXIV. p. 924. Fl. XXXIV, 18TS. 

 ^ Id. 1., n:o 172, XLIII, p. /Oo,' 1.S87. 



