558 P. CHALMERS MITCHELL. 
Relations of Thelaceros. 
Notwithstanding the recent labours of Hertwig and Andres, 
there is still much to seek in Actinian classification. In the 
great subdivisions there is, however, not much difficulty. 
Hertwig (8) criticises Andres’ erection of Stichodactyline and 
Thalassianthine into groups parallel to Actinine. He points 
out that the leading and obvious feature on which the primary 
division of Actiniaria must be made is mesenterial arrange- 
ment. Andres, however, in his synoptic table (loc. cit., p. 88) 
characterises a subdivision of the Actiniaria without naming 
it, as follows: ‘Colonna liscia, rugosa e solcata; se ha in- 
vezione queste non sono mai meno di dodici.” 
This unnamed division corresponds with Hertwig’s Hex- 
actiniz, and is parallel to the Edwardsine, Zoanthine, &c. To 
this large group thus based on a typically hexamerous arrange- 
ment of tentacles and mesenteries Thelaceros obviously belongs. 
But to determine its position within the group is more difficult. 
Hertwig alone has completely investigated the anatomy of any 
large number of Hexactinians, and the leading feature of a 
true classification he shows to be tentacles, pedal disc, acontia, 
and sphincter. From the ‘Challenger ” material he was able to 
characterise definitely a large number of families. But there 
still remain a number of forms, chiefly tropical, and chiefly 
characterised by their abnormal tentacles. For these Hertwig 
points out the need of further accurate investigation. Verrill 
(1 and 2) had previously grouped such forms under the family 
Thalassianthidz, and, besides pointing out their possession of 
two kinds of tentacles, first made the important observation, 
overlooked afterwards by Andres, and even by Moseley, that 
more than one tentacle may communicate with a single radial 
chamber. Andres relies entirely upon the tentacles, and 
divides the group corresponding to the Hexactinia thus :— 
