646 PERTSCHOECHINIDAE. 



membrane fully developed with all its plates, cuts, and carrying a prolon- 

 gation of the ambulacral system as in the adult. It requires but a slight 

 stretch of imagination to reduce our young Cidaris to a Palaechinus. Ob- 

 literate the two rows of coronal plates and we have a spherical urchin com- 

 posed of a series of plates corresponding in every respect to the hypothetical 

 Palaechinus. It consists, namely, only of an abactinal system (the large size 

 of the plates of Cidaris is similar to Palaechinus). of an ambulacral and inter- 

 ambulacral system, composed of hexagonal and pentagonal plates, perfectly 

 flexible, and of an oral opening through which we find teeth projecting. We 

 have thus a complete reconstruction of the typical Palaechinus. Let us now 

 examine in detail the various parts and trace the coincidence still further : 

 the Palaechinidae arc Cidaridae where the coronal plates are wanting (those 

 carrying the large tubercles and spines) ; the test, reduced to the abactinal 

 and actinal systems, being entirely made up of the flexible parts of the buccal 

 membrane of Cidaris. which is capable of great mobility, thus confirming 

 the view taken by Meek and Wort hen of the probability of considerable 

 adaptation of shape, without rupture, of parts of the test, as would be the 

 case in Echini when subjected to pressure. 



This view has received great support from the discovery of the flexible 

 Echinothuriae. in which the limestone coronal plates are only partly solidi- 

 fied, leaving considerable freedom of motion. The Echinothuria described 

 by Woodward in the Geologist for September, 1863, is undoubtedly the cre- 

 taceous representative of the deep-sea Asthenosoma (Calveria. W. Thorns.), 

 of which we owe a short description to Grube. Asthenosoma has also been 

 dredged by Pourtales and subsequently by Thomson, who has figured it in 

 his Depths of the Sea. I had previously figured a fragment in Part II. of 

 the Revision of the Echini, and have given a full description and figure of 

 the same species in the Ilassler Expedition Echini. In all these Echini (As- 

 thenosoma, Echinothuriae. and the Palaechinidae), the whole test is imbri- 

 cated ; the ambulacral and interambnlacral plates lap in opposite directions 

 in the Palaechinidae, as they do in the recent species of Echinothuriae. In 

 this family (Echinothuriae) we find the imbricating actinal membrane so 

 closely connected to the coronal plates, that there is no reason why we should 

 not have in Palaechinidae, as we have in Clypeastroids. the actinal mem- 

 brane reduced to an insignificant member, the coronal plates almost reaching 

 the jaws, or the test composed entirely of plates to be homologized with 

 those of the actinal membrane of Asthenosoma and Cidaris. 



