92 SVEN LOVÉtv, ON THE ECHINOIDEA DESCRIBED BY LINN/EUS. 



dually yields, its protuberances disappear almost entirely and 

 their former presence is betrayed solely by tlie red tints near 

 the tubercles and on the aboral half of the mighty cones. On 

 the ambiilacra it holds its ground better, and its protuberances, 

 tliouo-li much reduced, still remain on the uppermost plates. 

 It is from tliis their tenacious resistance tliat arises the dis- 

 ordered sequence of the upper ambulacral tubercles in most 

 species of the Pacific Ocean type of the genus. 



On the intimate structure of the epistroma I have but 

 little to say. Whenever a spine or a pedicellaria is severed 

 from a tubercle, its part of the cuticular envelope goes with 

 it, and the base alone of the naked cone is found covered 

 with what is left, Tub. 8, fig. 7, 8; Tab. O, Jig. 7. Of this 

 thin, external, continuous covering the ridges and protube- 

 rances are calcified portions, overlying the matrix-layer of the 

 tubercles, by the slow eruption of which they are subverted 

 and caused to trausform. On the secondary large nodular 

 bodies of the adult an assemblage may be seen of delicate 

 prickles, Tab. 8, fig. 6', the same perhaps that gives rise to 

 the appearance of connecting fibres in the iuterstices between 

 the ovate nodules of the protuberances, Jig. 3. The intimate 

 constitution of the calcified substance is different from that 

 of the plates and of the tubercles. When seen by reflected 

 light it presents widely extended, deviating systems of parallel 

 exceedinslv delicate lines which come into view successively 

 at different depths imder the surface, Tab. 9, Jig. 6. 



In the foregoing the epistroma has been described as it 

 appears on the test itself of the Arbaciae. It may be que- 

 stioned whether there is not somcthino- akin to it to be seen 

 on the spines. These, it is well known, present three forms. 

 Superiorly they are, on the interradia and to a greater extent 

 on the ambulacra, short and thick and slightly bent, then, 

 towards the ambitus, they become aciciilate, rather long, slender 

 or of moderate strengtli, and below the ambitus they are all 

 shorter and more or less fiattened. These ventral spines, the 

 first developed, are lined at the top with a thin giossy co- 

 vering first described by Desmoulins ^), abbreviated and smooth 

 on the dorsal side, ventrally descending a little way, with 

 ribs determined by the striation beueath, but more or less 



') Etudes, p. 3ö, 307; 1835—1837. - Actes Soc. Linn. Bordeaux 

 XXVII: 1869. 



