DISTRIBUTION IN TIME. 183 



and then attain the large size he has observed in the specimens found free in the shale, for he has 

 there found tlieiu in all stages of growth, from small rounded bodies, not larger than a pin's head, 

 to bodies of nearly half an ineh in length. 



Whatever these bodies may be, it is plain that Dr. Nicholson's account of them is irre- 

 concilable with the supposition that they represent either the gonangia or the gonophores of a 

 hydroid ; for, apart from their supposed development after detachment from the colony, their 

 origin fi-Qui the walls of the denticle is alone decisive on this point. Indeed, their connection 

 with the graptolite appears to be purely accidental. 



Ilall has called attention to the occurrence in the same beds which contain the graptolilcs 

 of minute free bodies, which he regards as the young or "germs" of the graptolites.' In their 

 earliest form they would appear to consist of a little chitinous oblong sac traversed longitudinally 

 by a slender chitinous filament, which is continued for a little way at both ends beyond the sac, 

 while at one end it is accompanied by two minute lateral spine-like processes. 



This early form has been traced through more advanced stages, in which it has been seen to 

 become more and more elongated, to develop denticles along its length, and finally to attain a 

 form in all essential points identical with that of an adult graptolite. 



Others slightly differing in shape from those described by Hall have been also obtained. 

 Indeed, these young graptolites— for there is no doubt that Hall is right in his interpretation of 

 their nature — are now well known. They are by no means uncommon in graptolitic shales, in 

 some examples of which I have seen them abounding in coimtless multitudes. 



Hall believes that he finds evidence of their having been contained within the so-called 

 reproductive vesicles of the graptolite. From his account of their relation to these, however, I 

 can recognise nothing but accidental proximity ; while if we admit that he has grounds for this 

 belief, we should then have, in the advancement of the embryo to a stage in which it has become 

 covered by a chitinous perisarc previous to liberation, a state of things quite at variance with all 

 we know of the reproductive phenomena of living hydroids. 



But little remains to be said regarding other views which have been from time to time 

 advanced as to the affinities of graptolites. These fossils have been compared to the chambered 

 shell of a cephalopod, a view, however, which in the present state of our knowledge of them need 

 not now detain us for a moment. They have also been compared to the selerobasic Actinozoa, 

 and their nearest living allies have been sought for in Virgularia and Fennatula. It is plain, 

 however, that the resemblance here is of the most superficial kind, and that the internal solid 

 smooth axis of a Virgularia or a Pennatula has nothing in common with the membranous test 

 of the graptolite, whose axial canal and tubular offsets point to totally different affinities. 



They have also been compared to the internal solid skeleton of larval echinoderms, an 

 approximation which has nothing in its favour, and which the occurrence of irregularly-branched 

 graptolites which have been continuously followed in the shale even for several feet will alone 

 render impossible. 



Their alleged polyzoal affinities have much more claim on our acceptance. Indeed, were it 

 not for the discovery of the graptolite gonosome (corbulas ?) we should have nearly as much to 

 say for this view as for that which would refer them to the IIvdroida, more especially as the 



' Ilall, op. cit., p. 33, pi. B, fii;s. 12 — 19. 



