182 PAXA^lIC DEEP SEA ECHDa 



posterior lateral ambulacra. In Pahopneustes Heminffi Arul. similar traces 

 of the primary tubercles are figured by Alcock.* 



The shapes of the odd interambulacral plates of the actinal side of the 

 test of Linopneustes and Paleopneustes are strikingly different. Those of 

 Paleopneustes, Fig. 266, are more Ananchytid, that is, more regular in size, 

 and although in Paleopneustes the plates of the actinal plastron and the 

 first pairs of plates of the lateral posterior interambulacra are large and 

 somewhat Spatangoid (Pis. 95, 97), yet they have not attained the great size 

 of the corresponding plates of Linopneustes, Fig. 267, where the first pair 

 of plates of the posterior lateral ambulacra occupies nearly the whole area 

 between the posterior lateral ambulacral zones and the ambitus (PI. 92). 

 On these plates, as well as on the sternum, the radiating arrangement of 

 the zones of the large primary tubercles has quite assumed the aspect of 

 recent Spatangoids and has not retained the more primitive arrangement 

 of the interambulacral primaries existing in Paleopneustes (Pis. 95; 97). 



In Linopneustes the primordial lateral interambulacral plates are small 

 and have also been greatly distorted. Figs. 274, 275, from the excessive 

 development of the first pair of plates following it (PI. 92) ; the primordials 

 are comparatively large and regular in Paleopneustes (Pis. 95 ; 97), though 

 excluded from the actinal system in P. cristaius in the older stages of growth 

 examined (PI. 96, figs. 1, 2). 



The primordial plates of the posterior lateral interambulacra are excluded 

 from the actinal system only in a few Spatangoid genera, such as Moira, 

 Faorina, and Micraster ; in the other Spatangoids they reach the actinal 

 system, though these plates are often reduced to the merest narrow slips 

 wedged in between the ambulacra. 



The phyllodes are perhaps better developed in Paleopneustes (Pis. 95 ; 97) 

 than in Linopneustes (PI. 92). They are specially prominent in P. hyslrix^ 

 where they are as prominently petaloidal as in many of the regular Spatan- 

 goids, in which the actinal system is greatly developed laterally and quite 

 crescent-shaped (Pis. 95; 97) compared to the shorter and broader actinal 

 system of Linopneustes (Pis. 92 ; 94, figs. 2, S). 



The shape of the labium gives us excellent characters to distinguish the 

 Spatangoids allied to Linopneustes and Paleopneustes. In Linopneustes, 

 Fig. 268, the labium is long, very narrow, the greater part of it flanked by 

 the lateral ambulacral plates 2 and 3 (Fig. 267). In Paleopneustes, Figs. 269, 



1 A Naturalist in the Indiau Seas, 1902, fig. "22, p. 168. 



