114 DESCEIPTION OF THE HAED PAETS OF SOME 



shaped rows as they approach the edge and lower surface of the arms ; 

 the sharp spines often become quite blunt in larger specimens. On the 

 actinal surface the spines are longer and sharper, usually arranged in lines 

 parallel with the longitudinal axis of the plates upon which they are 

 carried (PI. XVIII. Fig. 2). They gradually increase in size towards the 

 ambulacral furrow ; the spines of the interambulacral plates are still longer, 

 and those which form the actinal papillas attain the greatest develop- 

 ment (PI. XVni. Fig. 3). The above features this genus has in common 

 with Sblaster, differing from it, however, in not having in the interbrachial 

 angles the sharp line of demarcation between the arrangement of the 

 plates and rods forming the actinal and abactinal surfaces. The genera 

 differ also greatly in the structure of the interbrachial arch. In Cri- 

 brella the arch is well developed (PI. XVIII. Fig. 7), starting from 

 the angle of the arms and extending the whole way, between the 

 two floors, towards the actinal ring, while in Solaster the arch is lim- 

 ited to a free loop, swinging between the abactinal surface and its 

 basal interbrachial plates at the actinal ring in the interambulacral space. 

 The last actinal joint of the ambulacral system is large, the ambulacral 

 plates distant, and the interambulacral plates prominent, with a wide 

 actinal face, upon which are placed numerous spines of different sizes, 

 arranged in rows at right angles to the ambulacral furrow (PI. XVIII. 

 Fig. 2). 



On the actinal surface two to four water-tubes pass through the free 

 space enclosed by the limestone rods ; the water-tubes on the actinal sur- 

 face are less numerous, but longer. 



Astropecten articulatus. 



Astropecten articulatus M. T. 1842. Syst. d. Ast. 



Aslerias articulatus Say, 1825. Journ. Acad. Nat. Scien. Phila. 



PI XIX. 



On account of the great prominence of the marginal plates of the 

 actinal and abactinal surfaces in this genus, the limestone network is 

 reduced to a small surface. This is particularly the case on the actinal 

 surface, where the reticulation corresponding to the actinal surface of 

 the arms is reduced to a few minute plates between the interambulacral 

 and marginal plates placed at the angle of the arms near the base of the 

 jaws (see Figs. 4, 7, PI. XIX.). The remainder of the lower side of the 



