ME. P, H. CAEPEKTBE ON THE GENUS ACTINOMETEA. 451 



more than half that on an anterior arm, but they also attain a very 

 much greater size, the basal and median pinnules of an anterior 

 arm being very much less swollen than the corresponding pin- 

 nules of a posterior arm. A similar inequality in the develop- 

 ment of the genital glands has been noted by Agassiz as occur- 

 ring in the Echini *. 



The external appearance of the centrodorsal piece otActinome- 

 tra is very characteristic : like the cirri vrhich it bears, it is far 

 more constant throughout a considerable range of species from 

 very various localities than it appears to be in the individual 

 members of a single species both of the European and of some of 

 the foreign Antedons even when collected in the same locality. 



In Actinometra the centrodorsal piece is almost invariably a 

 flattened, circular or rudely pentagonal disk, somewhat hollowed 

 in the centre, and with low sloping sides marked out into distinct 

 sockets for the articulation of the cirri, which are limited to its 

 margin, the central portion of the plate being entirely free from 

 them. There is usually only one row of cirrus-sockets at the 

 margin of the plate; but in the large A. rohusta there maybe 

 two, and even traces of a third. In the typical forms oi A. poly- 

 morpha the number of cirri existing at any one time seems to vary 

 between 15 and 25 ; and the size and number of their segments 

 are tolerably constant, which is by no means the case in Antedon. 

 Further, the individual segments do not acquire the characters of 

 maturity at any thing like such an early date as they usually do in 

 Antedon, but even after the cirrus has attained a considerable 

 size and has the normal number of segments the latter remain of 

 a very rudimentary character, which is a somewhat exceptional 

 mode of development in Antedon. 



Another very marked difference between Antedon and Actino- 

 metra consists in the fact that in the latter genus the planes of the 

 external or distal faces of the first radials are parallel to the ver- 

 tical axis of the calyx, and not inclined to it at a considerable 

 angle as is the case in Antedon ; so that the whole of the ventral 

 surface of the calyx is in one horizontal plane, while in Antedon 

 the second and third radials and the bases of the arms are at a 

 much higher level than the pentagon of the first radials, owing 

 to the inclination of the distal articular faces of the latter. 



The most interesting point in the skeleton of Actinometra is 

 the condition of the rosette or metamorphosed basals, which re- 

 * 'Eevision of the Echini,' pt. iv. pp. 680, 681. 



