KONGL. SV. VET. AKADEMIENS HANDL. BAND. 19. N:0 7. 61 



bring out strongly tlie great contrast exhibited in this point by the Pourtalesiadaj. For 

 while these, in other regards, have not a little in common with the Spatangidas, and 

 share a few features with the Cassidulida?, they differ vvidely from either in being 

 homoiopodous. Their pedicels are all simple, and differ in size only, the phyllodean 

 and the upper trontal pedicels being larger than the rest, which are very minute, 

 FL IV, fiff. 16, 21, 22; VI, 40, 4U VII, 50. They all terminate in a rounded or 

 slightly tiimid top, which, in some states, is surrounded by a narrow circular brim, 

 not unlike that of Rhynchopygus, Pl. XI, fi,g. 118, 119. The very dense pigment of 

 their tissues so obscures its structure that I could not even make sure of the existence 

 of calcareous spicules. The peripodia, PL I, fig. 5, 6, 7 ; IV, 15, 24; V, 27, 29; VI, 

 44; VII.I 47; XII, 149, not unlike those of the Cassidulidte, are sunk, and the two 

 perforations confluent in the phyllodean and frontal ones, separated in the minute 

 subanals of V h. In the ventral, subventral and lateral plates of the ambulacra the 

 diminution of the peripodium relatively to the entire surface of the plate is carried 

 to the extreme, it being hardly discernible över the greater part of the ambulacrum, 

 and far inferior in size to the smallest of tubercles. The ambulacra are also all ape- 

 talous and perfectly even with the perisome, and all this, combining with peculiarities 

 of the interradial areas described above, tends to soften doAvn the elsewhere salient 

 diversities of the two pi-edominant systems, and to give to the whole surface of the 

 skeleton the character of smoothness, which the Pourtalesiada3 have in common with 

 their fellow-habitants of the great depths, the Apetalous Spatangidas. 



IV. THE CALYCINAL SYSTEM. 



The horaologies oC the calycinal system in Criuoidea and Echinoidea. Tiarechinus. Salenia. Its modi- 

 fications during geolof;;ioal development. Echinoconidre. Spatangidte. Its decay in the Pourtalesiadie. 



Years ago it occurred to me, as it had to others, that the general resemblance 

 of the »apical» system in the Cidaridge, Saleniada3 and Echinida;, to the calyx of cer- 

 tain Crinoidea, might be a morphological fact of importance with regard to a true 

 perception of the homologies of the skeletal constituents in the Echinoderms generally. 

 For such is in reality the conforrnity between the respective parts of both structures, 

 that, when ouce perceived, it must leave a strong impression of some hidden meaning 

 well worth understanding, and often enough it may have called forth reflexions that 

 far less often were recorded. Louis Agassiz *) once remarked, as of peculiar interest, 

 »the correspondence between the development of the calcareous central network» of 

 the disk in the young »Starfish and the stem of Pentacrinus»; the arrangement of the 

 tive plates »surrounding it and those alternating with them that will form the five 



') Twelve Leotures on ooinparative Embryology delivered before the Lowell Institute, in Boston, December 

 and January 1848—49. Boston, Manders & Co. 1849; p. 17, 22, 24, 25. 



