62 MR. p. 11. CARPENTER ON THE GENTJS ACTINOMETRA. 



it of only one or two whorls of marginal cirrhi, seem to be very characteristic of Act'mo- 

 metra ; aud it would be almost possible to distinguish the Antedons from the Actlno- 

 metrcB among the Comatulce described in Midler's memoir, by simply referring to his 

 descriptions of the " Knopf." 



"When the centrodorsal is viewed i?i situ, with all the cirrhi attached around its sides, 

 its central flattened surface appears almost circular ; but when the cirrhi arc removed, 

 so as to expose the low and more or less sloping sides to which they are attached, the 

 outer margin of the plate is seen to have a distinctly pentagonal form. This is well 

 seen in the large Act. robusta (PL V. fig. 15), with its three rows of cirrhus-sockets ; but 

 in Act. Solaris (PI. V. fig. 1) the angles of the pentagon are more rounded off, and there 

 is only one row, a complete one, however, of cirrhus-sockets, while the plate itself is of 

 a considerable diameter, so as entirely to conceal the first radials. 



In a variety of this species, which I believe to be identical with the Act. pectinata of 

 Retzius and Miiller, the diameter of the centrodorsal (PI. V. fig. 6) is very slight, so that 

 the greater part of tlie superjacent radial pentagon is visible outside its pentagonal 

 margin ; and there are only ten distinct cirrhus-sockets, two at each angle, though one 

 of the angles (the U2)per one in the figure) is marked by the presence of a third socket, 

 which cither indicates the commencement of a second row of cirrhi, or, and more pro- 

 bably, the incomplete obliteration of a pre-existing row. 



In Act. ])oIymo)'pha the angles of the pentagonal centrodorsal plate, the diameter of 

 which varies from 3 to 5 millims., are sharp and distinct (PI. VI. fig. 2) ; in varieties 3 

 (fig. 20) and 4, the dorsal and ventral surfaces almost meet at the edge of the plate, 

 Avhich is very slightly truncated, and marked in three places by the large sockets ( T7) 

 for the attachment of the few remaining cirrhi, Avhile other small openings indicate the 

 former existence of others which have been since lost. 



In var. 2 (PI. VI. fig. IG) the diameter of the centrodorsal plate is very small, as in 

 Act. pectinata (PI. V. fig. (!) ; and, also as iu this species, it normally bears ten cirrhi, two 

 at each angle. The sockets for the cirrhi are, however, but slightly marked, so that the 

 edge of the plate is but little truncated, aud it can hardly be said to have distinct sides 

 as in Act. solans (FL V. fig. 1). 



In the type, on the other hand (PI. VI. figs. 1, 2, 7), and more especially in var. 1 

 (fig. 14), in Avhich there are 25 cirrhi, the edge of the plate where the dorsal surface 

 passes over into the ventral is obliquely truncated, so that the plate has distinct sides, 

 which are marked by numerous cirrhus-sockets, as in Act. Solaris (PI. V. fig. 1). Towards 

 the ventral surface the angles of the pentagonal margin of the plate are prolonged into 

 five short processes (PI. VI. tig. 14, t), each situated between a pair of cirrhus-sockets ( U). 

 Their distinctness varies in different individuals ; they are especially marked in var. 1 

 (tig. 14), and in that specimen of the type which resembles it iu having 25 cirrhi (PI. VI. 

 fig. 7), and they are best seen after removal of the centrodorsal plate from the radial 

 pentagon which rests upon it. They exist also in the other varieties of Act. p)olymorpha 

 (PI. VI. figs. IG, 20), though they are not so distinctly visible externally, owing to the 

 greater extension of the dorsal surface of the plate than is found in the typo and iu 



