38 THE VOYAGE OF H.M.S. CHALLENGER. 



would have remained unknown, as the characters of the calyx in the recent species have 

 not yet been sufficiently studied to give any satisfactory clue to the detection of their 

 fossil representatives. 



Most of the fossil Comatulae have more or less well defined basals appearing 

 externally, which have not undergone metamorphosis into a rosette as is the case in very 

 nearly all the recent forms ; and it is probable that species like Antedon tessoni and 

 Antedon orhignyi, although they show no basals externally, will in reality prove to be 

 no exceptions to the rule. I feel some doubt, however, with regard to the Tertiary 

 species, only two of which arc represented by more than the centro-dorsal ; and this 

 affords but Uttle information respecting the presence or absence of a rosette. I am 

 inclined to think myself that in the matter of basals the Tertiary species resembled their 

 predecessors rather than their successors. But this view cannot be confirmed till the 

 discovery of a type which shows basals at the interradial angles of the calyx, or of one 

 in which these plates are visible on the under surface of the isolated radial pentagon. 

 But no Tertiary species of this kind are known, and neither Antedon alticeps nor 

 Antedon italica shows any traces of basals between the radials and the centro-dorsal. 



The determination of the generic position of a Mesozoic Comatula is often a matter 

 of considerable difficulty ; and this is especially the case when only the centro-dorsal is 

 preserved. In most fossil Cornatulse this part bears a considerable number of cirri which 

 are distributed over the greater part of its surface ; and it reaches a fair degree of 

 thickness ; so that there can be no doubt that these types have been correctly referred 

 to Antedon. But there are a few forms in which the centro-dorsal is relatively much 

 thinner and the number of cirri, which are almost or entirely limited to its sides, is 

 reduced. This is the case, for example, in two species from the Great Oolite and Bradford 

 Clay respectively, which I take to belong to Actinometra, rather than to Antedon. 

 Specimens which have the radials preserved can in some cases be referred to Antedon 

 without any difficulty, owing to the large proportion of height to width on the articular 

 faces of the radials. Such are Antedon sequimarginata, Antedon incurva, and Antedon 

 scrobiculata, the calyces of which closely resemble those of the typical forms of 

 Antedon figured on PL II. 



On the other hand, the generic identity of Actinometra loveni from the Gault is 

 equally indisputable. For there is no living Antedon yet known in which the centro- 

 dorsal loses all traces of its cirri and becomes separated from the flattened radial 

 pentagon by clefts at its sides ; while these changes are not uncommon in Actinometra 

 (PI. L VII. fig. 1 ; PI. LXV.). But in by far the greater number of Comatulas which have 

 the radials preserved, the height of these plates is quite small relatively to their width, as is 

 invariably the case in the living Actinometra (PL V.). When these radials rest on a thick 

 centro-dorsal which is marked by a number of cirrus-sockets [Antedon decameros, Antedon 

 greppini) there can be no question that the type in question belongs to Antedon. But 



