POURTALESI.E. ]21 



In Echinolampas (PI. 64, fig-, f)) and Conolampas tlie central plate of the 

 apical system rises like a button above the general level of the test, it is 

 thickly covered with small tubercles, many of which are glassy as in 

 Echinoneus. 



The ambulacra of Neolampas show the first trace of the wedging of 

 the ambulacral })lates (PI. 64, fig. G), one plate in each ambulacrum only, 

 which is carried to such an extreme in Conolampas, Rhynchopygus, and 

 Echinolampas as to obliterate the primitive regular arrangement of the 

 ambulacral plates as it still exists in Neolampas and in the young of Rhyn- 

 chopygus.^ The smaller wedge-shaped plate is the only indication of a 

 phyllodic expansion, as in Rhynchopygus,^ while in Echinolampas the 

 ambulacra are not thus expanded (PI. 65, fig. 3), but the two sides merely 

 diverge towards the ambitus in straight lines, though externally the pores 

 expand in Conolampas (PI. 65, fig. 5) and Echinolampas (PI. 65, fig. 1) 

 into a somewhat phyllodic outline. In the Pacific species of the genus 

 {R. pacifims) one of the first pair of plates of each ambulacrum, though large, 

 carries only a single pore, the ankylosis of the other plates showing it to be 

 composed of two plates. 



SPATANGID.Sj Agass. 



POURTALESIiE A. Ag. 



It is interesting to trace the gradual modifications in the odd posterior 

 interambulacrum from the simple conditions such as exist in Urechinus 

 (PI. 73, fig. i), where the primordial interambulacral plates all reach the 

 actinal system, Fig. 159, but in which the sternum consists of a single 

 elongate polygonal plate following the primordial, the labium differing in 

 Ro way except in size from the single plate following the primordial of the 

 other interambulacral areas. We may thus look upofi Urechinus, in which 

 the plate next to the primordial is single in all the interambulacral areas, 

 as showing us the probable origin of the single sternal plate in some 

 Spatangoids, while in the Spatangoid types such as Collyrites, Echinoneus, 

 and the Cassidulids, in which the primordial plate is followed by two plates 

 in all the interambulacral areas, we have the prototype of the sternum 

 made up of two plates which, from the great extension of the posterior 



1 Loven, :^tudes PI. VII, fig. 61. « Lov^n, fitudes, PI. VII. fig. 67. 



