MUSEUM OF COMPAEATIVE ZOOLOGY. 



163 



in which there is a considerable amount of anambulacral plating in the 

 anal area ; but it is usually rather of the nature of an aggregation of 

 tubercles than of a regular pavement of plates, and I have never met 

 with it so completely covering the disk and extending out on to the arm 

 bases and lower pinnules as it does in Antedon. The species in which I 

 have found it to reach its greatest development is a large one which is 

 common at Cape York, and is probably identical with Act. robusta 

 Liitken, MS. The plating supports the sides of the ambulacra on the 

 disk and occupies the intervals between them, the plates immediately 

 adjacent to the grooves being extensively pierced by the water-pores. 

 The plating ceases, however, just within the circumference of the disk, 

 so that the perisome of the arms and pinnules is perfectly bare, just as 

 in the common Antedon species which inhabit the North Atlantic. 

 This entire absence of side plates and covering plates on the arms and 

 pinnules of Actinometra, even in species which have a strongly plated 

 disk, is a very singular peculiarity, and one which I am quite unable to 

 explain. 



The principal differences between Antedon and Actinometra are con- 

 veniently summarized in the subjoined table. 





Antedon. 



Disk with central or subcentral mouth 

 and five equal ambulacra. 



Oral pinnules not specially distin- 

 guished. 



All the arms equal in length, grooved, 

 and tentaculiferous. 



Red spots (sacculi) always present at 

 sides of the ambulacra. 



" Sense organs " wanting. 



Pinnule ambulacra may be protected 

 by side plates and covering plates. 



( Sirrhi many, sometimes very numerous, 

 and more or less covering the under sur- 

 face of the hemispherical, conical, or co- 

 lumnar centrodorsal. 



Outer faces of radials relatively high, 

 with large muscle plates, and much in- 

 clined to the vertical axis of tin- calyx. 



Actinometra. 



Disk with eccentric mouth and a vari- 

 able number of unecpial ambulacra, at 

 least two of which enclose the anal area 

 in a horseshoe-shaped curve. 



Oral pinnules have terminal combs. 



Some of the hinder arms may be much 

 shorter than the rest, ungrooved, ami 

 non-tentaculiferous. 



Sacculi wanting. 



Brown spots (sense organs ?) may be 

 present on the dorsal side of the pinnule 

 segments, mostly in the ungrooved hinder 



arms. 



Pinnule ambulacra (when present) un- 

 protected by plates. 



Cirrhi few in number, and aim. 

 tirely limited to the margin of the iis- 

 coidal centrodorsal. 



Outer faces of radials relatively wide. 

 with small muscle plates, and nearly or 

 quite parallel to the vertical axis of the 

 .ah \. 



