96 DESCRIPTION OF THE HARD PARTS OF SOME 



ABterias ochracea. 



Asterias ochracea Br. 1835. Prodrom. 



PI XL 



The striking differences which apparently exist on a cursory examina- 

 tion of the species allied to A. ochracea are not found to be of suffi- 

 cient importance, when analyzed, to warrant us in considering the genus 

 Pisaster, as recognized by Professor Agassiz, anything more than a con- 

 venient systematic generic subdivision ; the special points of difference are 

 the great width of the ambulacral system, its elongated plates, the breadth 

 of the farrow forming the median ambulacral ridge (seen from the 

 interior) (PI. IX. Fig. 5), the proximity of the openings for the passage 

 of the ambulacral tubes on each side of the median ridge, with the cor- 

 responding slender interambulacral plates carrying only one row of long 

 spines at the outer extremity. When denuded of spines the reticulation 

 of the actinal surface of the arms adjoining the interambulacral plates 

 forms a close pavement with small interstices (PI. XI. Fig. 4); the tubercles 

 carrying the spines are arranged in three or four rows at right angles to 

 the longitudinal axis of the arm ; they have a deep slit at the top of 

 the boss ; these tubercles are connected laterally by a comparativeh' low 

 ridge. In the reticulation of the abactinal surface of the arms the 

 primary spaces are quite large, but these are greatly subdivided by a 

 secondary system (PI. XI. Figs. 1, 2) (more or less prominent), consisting 

 of smaller plates, most irregular in shape, which encroach upon the 

 primary areas and subdivide them again, or materially reduce the area 

 through which the water-tubes can be protruded. The reticulation of 

 the actinal surface carries large club-shaped spines of moderate length, 

 while the spines of the upper surface are shorter but similarly shaped, 

 presenting the appearance of having been ground down so as to form 

 nearly continuous walls on the separating ridge of the reticulations (PI. 

 XI. Fig. 1). The interambulacral papilla3 are generally cylindrical, some- 

 times pointed or somewhat club-shaped at the tip, contrasting with the 

 generally ' flattened and slightly spatulate interambulacral spines of 

 Asteracanthion proper. Tlie interbrachial partition (PI. XI. Fig. 5) is 

 naturally very well developed, owing to the great number of narrow 

 interambulacral plates from which the brachial reticulations arise. The 

 whole reticulation of the arms rs far more solid than in any other 

 group of species of Asteracanthion (Asterias); compare PI. IX. Fig. 0, the 



