54 PAL&OTEOPUS JOSEPHIX.E. 



There is little difference in the arrangement of the ambulacra] plates in 

 older or younger specimens. The two large genital openings, already present 

 in the youngest specimens known, have greatly increased in size, and the 

 madreporic openings are better developed in larger specimens (PI. XXIII. 

 Fig. 12). The mode of formation of what has been called a compact abaeti- 

 nal system is admirably shown in the structure of the apex of Palaeotropus, 

 in which the exterior sutures of all the plates adjoining the interambulacral 

 areas are most distinct, while the interior junction of the genital plates has 

 become completely obliterated (PI. XXIII. Fig. 12). they being still sepa- 

 rated by the odd imperforate genital plate which is intercalated between 

 them, and of which the posterior and lateral sutures are still well marked, 

 while the interior anterior sutures are no longer visible. In Pahvobrissus 

 the sutures of all the plates of the apex can be distinctly detected. 



In the youngest stages, as well as the larger, we find near the actino- 

 stome in each ambulacra! area one or two unusually elongate sphaeridia. 

 They have a comparatively long reticulated stem, and it is interesting to 

 note that in some of the larger specimens we find near the ambitus on the 

 actinal side peculiar miliary spines which I am inclined to consider as modi- 

 fied sphaeridia. These peculiar spines have the shaft of a miliary, but the 

 tip is swollen out to a clear sphere resembling in all respects the head of 

 sphaeridia, of which the base of attachment is usually merely rudimentary, as 

 described by Loven ; in addition we have such types as the sphasridia typical 

 of the actinal side of the genus Pakvotropus. This would seem to show 

 that the sphaeridia, like the pedicellariae, the spines of the fascioles, and 

 perhaps other appendages of the test of Echini, are only modified forms of 

 spines. 



Danielssen and Koren * have figured peculiar spines of Hymenaster, 

 which also throw considerable light on the nature of the sphaeridia. They 

 are clusters of reticulations at the base of a central shaft, much like the 

 reticulations figured by Loven as characteristic of the base of sphaeridia. 

 These spines arc surrounded by a bag-like pouch representing undoubtedly 

 lor Starfishes the bag-like appendages of certain spines of Asthenosoma and 

 Aspidodiadema, which in their turn seem to have close affinities with the 

 peculiar gland of certain Echinid pedicellariae described by Sladen and 

 others, as 1 have already noticed in the description of some remarkable 

 spines occurring on the test of Aspidodiadema. 



* Plate II. Figs. 2, :;. Nyt Mag. for Naturvid., XXVIII. 



