20 S. LOVEN, ON POURTALESIA, A GENUS OF ECIUNOIDEA. 



sister-branch. And in the whole course of its development it continues to deviate 

 more and mox-e. 



It begins with the Echinoneidaj. Galeropygus and Hyboclypeus, thti oldcst known, 

 from the Liassic and Oolitic periods, with a rounded auibitus, have the excretorj' 

 opening close to the calyx; in Galeroclypeus, Pachyclypeiis, Desorella, from the Middle 

 Oolite, it is posterior, even marginal. In the Oolitic species of Pyrina it is near the 

 calyx, or at the middle between it and the posterior margin; in the Cretaceous species 

 it is marginal, and it becomes ventral in Echinoneus, Tertiary and recent. 



Among the Cassidulidae the Oolitic genera, Clypeus and Pseudodesorella, have 

 the periproct not far removed from the calyx, and the gap is often filled by a pro- 

 longation of the radials I and V. Out of twenty Oolitic species of Echinobrissus nine 

 belong to the older division, and five of these have the periproct sub-calycinal, one 

 not far behind, and three at half the distance between the calyx and the postei'ior 

 margin; seven species are Middle Oolitic, and it is sub-calycinal in one of these, at 

 one fourth, one third, or one half of the distance in five, behind the middle in one. 

 In the Upper Oolite, of four species, one alone has the vent sub-calycinal, three 

 posteriorly in the middle, and the three Cretaceous species, as also E. recens, of a 

 lengthened general form, all have it posterior, at the middle, or even behind. In all 

 the numerous genera of the Cretaceous and Tertiary periods the periproct is from 

 supra-marginal to infra-mai"ginal, and in those of Tertiary origin ventral, even sub- 

 oral. In general, and with the sole exception of Pygurus, in which, in Oolitic time, 

 it reniained marginal or was even infra-marginal, the site of the periproct in the oldest 

 known forms of Edentates was in close vicinity to the calycinal system, and from this 

 point it retrograded farther and farther dui'ing the Cretaceous and Tertiary periods, 

 even so as at last to become ventral. 



In the CoUyrites, of Oolitic existence, the periproct is posterior, and so it is in 

 the Holasterida3. Among the earlier forms of the Spatangida3 its site is more or les. 

 dorsal, in those of låter appearance it recedes, according as the abdomen lengthenss 

 Like other features permanent in earlier forms, transitory in the more recent, this 

 pi"ogressive modification is exhibited in the course of the individual development of 

 recent forms. 



Philippi, when describing the antarctic Spatangi ^) for which Troschel afterwards 

 created the genus Abatus, relätes that he found, within their deepened petals, some 

 very minute specimens which he suspected to be their young, and this discovery has 

 since been confirmed and extended by Al. Agassiz ^), who examined specimens collected 

 at the Kerguelen Islands, as well as by Sir Ch. Wyville Thomson '^) and by Studee ^), 

 who both at that same locality had frequent opportunities of seeing live specimens. 

 I am indebted to the well-known liberality of the late Professor W. Peters for a 

 present, Avhich cannot be too highly appreciated, of three young ones taken out 

 of the marsupial petals of the unique specimen bearing such, in the Berlin Museum. 



1) Études, p. 81. 2) Wiegm. Arcli. XI, 1845, p. U7. ^) Proc. Amer. Acad., XI, 1876, p. 231. — Rep. 

 Eohin. Chall., p. 177, pl. XX a. «) Journ. Linn. Soc, XIII, p. 67. — Voy. Chall., II, p. 229. ^) Mon. 

 Ber. Berl. Akad., 1876, p. 457 — Ib. 1880, p. 881. — Zool. Anzeiger, 1880, p. 343. 



