ME. P. II. CARPENTER ON THE GENUS ACTINOMETRA. 17 



is equally applicable, that this last can only he used to designate the family, while one 

 of the two names, Antedon and Alecto, which have precedence over it in point of time, 

 has gradually become more limited in its meaning, and the other has ceased altogether 

 to be applied to the Crinoids. 



II. On the Characters of the Genus Actinometra. 



(§ 13) We have seen that while the distinction drawn by Midler between Alecto and 

 Actinometra depended upon the number of groove-trunks reaching the peristome, irre- 

 spective of the position of the mouth, the genus Antedon, as defined by Mr. Norman, 

 and as subsequently used, is distinguished by having the mouth central and the anus 

 lateral. 



There are, however, numerous species of Alecto in which, according to Miiller 1 , " die 

 Afterrohre nimmt die Mitte der Scheibe ein, so dass der Mund seitlich sesen den Rand 

 der Scheibe riickt ohne dass die Ambulacra ihre symmetrische Vertheilung auf die fiinf 

 Avmstamme einbussen." These forms have obviously no place in the genus Antedon, 

 while they were excluded from Actinometra by Miiller, who goes on to say, " In 

 andcrn abweichenden Arten geht die gleiche Vertheilung verloren, indem der excentriseh 

 liegende Mund weniger als fiinf Furchen der ambulacra aufnimmt, dann werden 

 einzelne dieser Furchen herrschend unci verasteln sich, indem sie einen grossen oder den 

 grossten Theil der Scheibe umziehen, auf mehreren Armstammen zugleich, so dass die 

 Scheibe von einem Furchenkreis umgeben ist, der jedoch an einer Stclle nicht gcschlossen 

 ist (Actinometra) (PI. I. figs. 2, 3, 5). 



In PL I. figs. 6-16 is represented the distribution of the groove-trunks or ambulacra 

 on the disks of the eleven specimens of Act. polymorpha which I have been able to 

 examine. A glance at these, no two of which arc alike, will suffice to show that within 

 the limits of one and the same species there may occur individuals, some of which would 

 have been referred by Miiller to Alecto, some to Actinometra, and some which, strictly 

 speaking, have no place in either of these genera. 



Thus, for example, the specimen represented in fig. 16 would probably have been 

 classed as Alecto by Miiller ; but although five groove-trunks leave the peristome, their 

 branches are by no means equally and symmetrically distributed to the different arms. 

 On the contrary, one of them gives off far more branches than any of the others, sup- 

 plying all the arms borne by two radii (I^-Eo), together with half of those of another 

 radius (AJ ; while another trunk running straight, out from the peristome bifurcates but 

 once, and only supplies two of the arms of one radius (Bo). Again, in fig. 15, only four 

 groove-trunks leave the peristome, one of which gives off a large number of branches, as 

 in fig. 16, also supplying all the arms of two radii (D^Ea). According to Midler's system, 

 therefore, this individual is an Actinometra . 



In all the other nine specimens of this species which I have examined, however, there 

 are invariably more than five groove-trunks running out from the excentric peristome 

 (PL I. figs. 6-11). Even in the small specimen with thirteen arms, represented in fig. 6, 

 there are six groove-trunks, while in fig. 11 there are eight, and in all the other figures 



1 Berlin Monatsberichte, 18iG, p. 177. 



SECOND SERIES. — ZOOLOGY, VOL, II. 3 



