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



known in the whole of the Gnathostomata as well as in the older Ateleostomata, ob- 

 scurely marked in the Holastridaj, manifest in the true Spatano;i; the odd ititerradium, 

 5, at first, in the Holastridte, nearly similar to the other four, has its ventral plates 

 separate, transversely triangular: Meridosterni, but later, in the higher genera, formed 

 into two large collateral sternal plates: Amphisterni; the fasciola^, a feature unseen 

 before: Adeti, at first are vaguely extant: Prymnadeti, or marginal alone, then the 

 peripetalous fasciola is added, till, towards the end of the Cretaceous period, the sub- 

 anal appears: Prymnodesmii, in the higher Spatangida; always traversing adorally the 

 sixth plate of I a and V b, and marking off the first five plates as ventral, the follow- 

 ino' as abdominal and dorsal; the spines are bristle-like, generally curved, tending 

 backward, where not otherwise directed by the fascioles; the pedicels, which in 

 Echinoneus are disciferous and similar to those of the Endocyclic Gnathostojnes, and 

 homotypic all over, in Cassidulus simple, in the petals only modified into branchials, 

 attain in the Spatangidaj their highest development as diversely adapted organs, 

 those of the phyllodes penicillate, the subanal mostly sub-penicillate, the lateral 

 simple, the frontal variously constructed, and those of the petals, where these exist, 

 branchial. 



While these and other changes are successively introduced in the structure of 

 the Spatangidge, the two main constituents of their skeleton, the interradial system 

 and the ambulacral, hold their own, and keep up their relative parts in the evolutio- 

 nal labour, their elements gradually assuming novel and almost refined forms ^). It is 

 not so with the third constituent, the calycinal system. 



When the Collyrites appear, the first of the Spatangida3, 

 and after them the Holastrid;p, the excretory opening is po- 

 sterior and distant, and the calycinal system in the condition 

 presented by some Echinoneida^ of the Oolitic period, as e. g. 

 by Hyboclypus. It is generally more or less lengthened, and 

 the only trace of the passage of the periproct is the total 

 suppression of the central disk and the costal 5. In this state 

 it remains in the Adete, Meridosternate forms: Anancites ^) 

 Offaster, Holaster, Cardiaster, Heinipneustes, introduced at the 

 beginning of the Cretaceous period and continued into the Eocene; 

 it is still seen, in the seas of the present time, at great depths, 

 "'""crcfnd ?yttm' ^''' i" ^hc Adete Cystechinus Al. Ac, and in Urechinus Al. Ag., 



PL XXL fig. 239—242, which is said to be provided with a sub- 

 anal fasciola. It is shortened, but tightly closed by the contiguity of the costals 1 and 

 4, in the forms with a true, but yet imperfectly developed sternum: Toxaster, Heter- 

 aster and Enallaster; it is short and subcircular, closed behind by the costals 1 and 4 



') Observe the contrast between the plain and rigid anguhirity of the plates in Micraster, Etudes, pi. 

 XXXIII, of Cretaceous existence, and the freedom of outline, even elegance, observable in corresponding 

 parts ill later Prymnodesmian types, Tertiary and recent, such as Echinocardium, pi. XXXIX; Plagionotus, 

 pi. XL, Breynia, pi. XLI, Maretia, pi. XLII, Lovenia, pi. XLIII. The contrast is less striking between 

 Hemiaster and the later Prymnadetes, ib. pi. XXVI — XXXII, tig. 197. 



2) Etudes, pi. XI, fig. 96, 97, 98. 



