16 THE VOYAGE OF H.M.S CHALLENGER. 



that we find the greatest divergence from the normal structure of the Sea-urchin spine, 

 while it is remarkable that among more recent genera such as the Spatangoids, and 

 especially the Clypeastroids, we find the simplest types of spines more nearly approaching 

 the spines characteristic of all the young Echinids of the present day. So that we may 

 be said to find at one extremity of the series spines with a complicated structure such as 

 that of the spines of the Cidaridse, and at the other the simple spines of the Clypeastroids 

 and Spatangoids. 



Among the Cidaridje we do not find either in the new species of Goniocidaris or in 

 Porocidans any important points of structure not previously noticed. Goniocidaris 

 florigera (PL XXXVIII. fig. 11) presents the characteristic Cidaris features of an outer 

 crust with a reticulation becoming gradually finer towards the central portion of the shaft 

 occupied by the " pith " (PI. XXXVIII. fig. llo). Such is also the general structure of 

 the spines of Porocidaris {P. elegans) (PI. XXXVIII. figs. 12-16) ; the reticulation 

 extending from the crust towards the centre is somewhat coarser than in Goniocidaris, 

 and the central space occupied by the irregular reticulation of the pith is comparatively 

 greater. In the sections of one of the large elliptical spines the crust is comparatively 

 thin, and four or five of the outer lines of reticulation occupy the space next to the crust, 

 the rest of the central space being filled with the pith (PL XXXVIII. fig. 15). In 

 Salcnia {Salenia hastigera) (PL XXXVIII. fig. 10) we find an outer crust as in the 

 CidaridiB, and the same coarse reticulation of the shaft towards the centre which we 

 find in Porocidaris ; the central pith is quite hmited in extent. 



In Ccelo2)leurus we have, as in the Cidarida3, a great development of the close inner 

 reticulation of the shaft (PL XXXVIII. fig. 7) with a ring of large foramina round the 

 central pith (see also section of spine of Ccelopleurus Jloridanus, Eevis. Ech., pi. ii.' fig. 

 15) with an outer crust much as in the Cidaridae and Salenidaj ; this is the structure of 

 the large curved primary spines. In smaller spines the central part of the shaft is com- 

 posed of larger reticulations and a more irregular reticulation (PL XXXVIII. fig. 8) ; 

 while in the smaller spines we find a very open central reticulation passing directly into 

 the ill-defined outer crust (PL XXXVIII. fig. 9). This structure of the spines closely 

 resembles that of the flattened spines of Podocidaris (Revis. Ech., pi. iv. fig. 15), in which 

 the outer crust is reduced to a very thin layer except where it forms the projections on 

 the extremities of the lines of well separated reticulating spokes. In Plate XXXVIII. 

 fio". 9, the line of demarcation between the outer crust and the coarse reticulation of the 

 shaft is indistinct, and it is only occasionally that in spines like these the outer crust can 

 be recoo-nised and seen to have the essential characteristics we find in the sections of 

 Plate XXXVIII. figs. 7, 8 ; so that the Arbaciadse, as far as the structure of the spine is 

 concerned, may be considered to form the passage between the Cidaridse and the 

 Echinidse, that is, they have an outer crust as in the Cidaridse and an inner pith, but the 

 spokes show a tendency to assume a rudimentary monocyclic arrangement, pomting to 



