MUSEUM OF COMPARATIVE ZOOLOGY. lfil 



The mouth is radial, and the disk either hare or more or less covered with 

 irregular calcareous concretions. Its diameter, in large specimens, is 12 or 15 

 millimeters, and the spread of the arms is about 25 centimeters. The color (in 

 alcohol) varies greatly, — white, straw-color, and brown, either alone or more 

 or less mixed with a dirty lilac. 



The results of my examination of the " Challenger " and " Blake " 

 collections, and of the numerous ComatuUv to which I had access in the 

 various European museums last autumn, entirely confirm and extend 

 the conclusions to which I had been previously led respecting the separa- 

 tion of Antedon and Actinometra as distinct generic types.* A glance 

 at the skeleton is sufficient to enable me to distinguish the genus ; and 

 it is even possible to determine the genus of a mere arm fragment, or 

 in fact of a single pinnule. For the problematical red spots (sacculi) 

 at the sides of the ambulacra which have puzzled all the workers on 

 Antedon are entirely absent in Actinometra. They are much more abun- 

 dant in some Antedon species than in others, but they are always to 

 be found by careful examination. They are also present in Ophiocrinux, 

 Promachocrinus, Pentacrinus, Rhizocrinus, Bathycrinus, and Atelecri/ws, 

 all of which are types with a subcentral mouth. But I have never yet 

 met with them in Actinometra, though some specimens of Ad. jndckella 

 seemed at first sight to be exceptions to this rule. Microscopic exam- 

 ination showed, however, that the appearances are due to irregular 

 aggregations of brown pigment at the sides of the ambulacra, which are 

 altogether different from the vesicular sacculi that are so puzzling in 

 Antedon and in the other genera mentioned above. 



On the other hand, there are certain structures which are peculiar to 

 Actinometra, though far from being as universal in their occurrence as the 

 sacculi are in Antedon. I allude to the brown cellular bodies which I 

 have supposed to be sense-organs. I found them first in some specimens 

 of Act. polymorpha from the Philippines,! and have since detected them 

 in two of the "Challenger" species, also from the Eastern seas. They 

 are confined almost exclusively to the middle and later pinnules of the 

 ungrooved hinder arms, each one occupying the dorsal half of a pinnule 

 segment just beneath its calcareous axis. There are several individuals 

 in a very large collection of Act. mcridionalu obtained at French Beef in 

 1869, which exhibit this peculiarity in a very marked degree. It also 

 occurs in a few individuals of the Cape Frio variety vl' this species, and 



* Actinometra. Trims. Linn. So.'.. See. Ser., Zool, Vol II. pp. 17-! 

 — Quart. Journ. Geol. Soc, Vol. XXXVI. pp. il-i:;. Proc R, s.. 1879, p. 394. 

 t Actinometra. Traus. Linn. Soc, Sec. Ser., Zool, Vol II. p. 10, PL 11. Kg. 6. 



