2 
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Leptometra. Moreover, the central column of cirrus sockets in the 
radial areas is in one of these species reduced to a single socket, the 
centro-dorsal appearing, therefore, very much like that of Leptometra. 
Psathyrometra is an inhabitant of fairly deep water, just as Lep- 
tometra is, and there appears to be little doubt that the latter 
originated from the former, or at least from the same common 
ancestor. Psathyrometra is an Indo-Pacific genus, like Mastigometra ; 
and so we must consider Leptometra, like Antedon, primarily an 
East Indian genus, having probably reached the Atlantic by way 
of the Mediterranean, and extended its range as far north as 
Scotland. 
The intrusion of the Mastigometra and Psathyrometra stock 
into the Mediterranean and thence into the Atlantic must have 
taken place very long ago, for we find their representatives (Antedon 
and Leptometra) now generically separable from the parent stock. 
As fossils in the rocks many genera (including two very like, if 
not identical with, these) are known in Europe and in England 
which are now confined to the East Indies, or at most to the 
Indo-Pacific-Japanese area, and it is quite possible that Antedon 
and Leptometra entered upon the European shores at the same 
time as the now fossil species of Comasteridæ, Zygometridæ and 
Himerometridæ, and that the changing conditions in the past 
geologic ages have gradually extirpated one by one the species 
and genera of all the families except the Antedonidæ, and of this 
family all the. genera except Antedon and Leptometra, which. alone 
were able to adapt themselves to the new environment. 
It is interesting to apply some of the ecological theories which 
have been recently proposed to that most interesting fossil: coma- 
tulid, the pelagic Vintacrinus. Mr. Frank Springer, who first 
suggested the probable relationship of this genus with the. Coma- 
steridæ, noticed that the young were rarely found with the adults, 
but were usually in separate masses by themselves. It has been 
urged that these smaller individuals represented in reality a different 
species, but there can be no doubt that Mr. Springer is right in, 
Vidensk. Meddel. fra den naturh. Foren. 1909. 9 
