HARD PARTS OF SOME NORTH AMERICAN STARFISHES. 



ASTERIAS. 



In the genus Asteracanthion (Asterias) the true character of the plates 

 of the abactinal and actinal surfaces is far more difficult to trace than in 

 other genera where the plates retain more or less homogeneous features. 



In Asteracanthion, although in the younger stages (as shown in Plate 

 VIII.) the reticulation consists entirely of plates readily distinguished 

 one from the other, yet in the adult the plates have become changed 

 to a mere irregular network anastomosing in all possible directions (PI. 

 IX. Fig. 3), and thus rendering it quite difficult, if not frequently impossi- 

 ble, to trace the connection of the actinal and abactinal reticulation with 

 the interambulacral plates. 



In the majority of species of this genus the plates adjoining the inter- 

 ambulacral plates are cross-shaped (PI. IX. Fig. 6), connecting with ad- 

 joining plates at three ends, in front, behind, and towards the abactinal 

 surface ; the other end connects with the interambulacral plates. These 

 plates lose their regularity as they ascend towards the abactinal side on 

 the edge of the arms, the prongs becoming gradually short processes, 

 and finally simply rods or irregularly shaped plates all more or less 

 imbricating (PI. IX. Fig. 4). The spines are generally attached to the 

 rods by a very shallow socket fitting into a rudimentary tubercle and ring. 

 The spines of the interambulacral plates are movable, those of the actinal 

 and abactinal less so, and frequently soldered to the reticulation. 



Asteracanthion berylinus. 



Asteracanlhion berylinus* Ag., A. Ag. 1863. Proc. Amer. Acad. Boston. 

 Asterias Forbesii Des. 1848. Proc. Bost. Soo. Nat. Hist., UI. p. 67. 



PL IX. 



The base of each of the interambulacral plates at its junction with 

 the ambulacral plates is marked by a pore for the passage of a water- 



* For the typography used to explain the synonymy, see A. Agassiz, Kevision of the Echini, p. 26_ 



