368 Prof. Allman on the Morphology 



pears as a free extension of tlie rod. Its distal extension 

 would then correspond to what had been the young growing 

 portion of the graptolite, as yet destitute of denticles and with 

 its perisarc so delicate as to be incapable of preservation in 

 the fossil, so that the thin perisarc has perished along with the 

 soft coenosarc it included, its thicker rod-like portion being the 

 only part preserved. 



This view is borne out by the fact that in the very young 

 stage of the graptolite a distal extension of the body along the 

 rudimental rod, and beyond the incipient denticles, may be 

 noticed ; while it is further confirmed by an observation by 

 Dr. Nicholson*, who tells us that in some specimens of Z>^}?fo- 

 graptus pristis he has seen the common canal without denticles 

 continued on each side of the prolonged rod. 



The continuation of the rod beyond the denticle-bearing 

 portion at the proximal end of the graptolite may also have 

 been accompanied by an extension of the coenosarc and its en- 

 veloping perisarc in this direction, tlie rod alone remaining in 

 the fossil. To this view an observation of Mr. Carruthersf 

 gives support ; for he has noticed the prolongation of the rod 

 at the proximal end of Clvmacograptus scalaris frequently in- - 

 vested for a short distance by a sheath. 



If this explanation be accepted, the continuation of the rod 

 as a naked filament beyond the denticle-bearing portion of the 

 graptolite need no longer surprise us. A comparison of the 

 rod with the chitinous spines which bristle over the surface of 

 Hydractinia may also here suggest itself ; but these spines are 

 not only invested by a coenosarcal layer, but are permeated by 

 canals which are lined by coenosarc, while in other respects the 

 approximation of the graptolites to Hydractinia offers too 

 many difticulties to allow of its being attempted. 



The lateral spines often present at the proximal end of the 

 graptolite seem to be of a different nature from that of the rod, 

 and would rather appear to be referable to the same group of 

 structures as the chitinous spines and variously formed pro- 

 cesses by which the hydi'othecfe and other/^parts of the perisarc 

 of living hydroids are not unfrequently ornamented. 



It has been already said that the advocates of the hydroid 

 nature of graptolites regard their calicles or hollow lateral 

 offsets as hydrothecse. If this be really the nature of these 

 parts, the mode in ^vhich their cavity opens into that of the 

 main tube is exceptional ; for in the living hydroids the point 

 of communication between the hydrotheca and tube of the 



* Geological Magazine, vol. iv. 1867, p. 261, note. 



t In the ' Intellectual Observer ' for June, 1867, p. 370. 



