Zoological Position of the Graptolitidte. 57 



Barr., but it is very largely developed in a new species oi Di- 

 plograpsus from the Dumfriesshire Shales, which I have named 

 D. vesiculosus (PL III. fig. 11). 



The homologies of the solid axis, with its extensions and ap- 

 pendages, are by no means clear. There is no known structure, 

 either amongst the Hydrozoa or Polyzoa, which could be looked 

 upon as its exact equivalent ; and it is probably related (but by 

 analogy only) with the horny or calcareous " sclerobasis " of the 

 Gorgonidaj and Pennatulida? amongst the Actinozoa. Its chief 

 function certainly seems to have been to give due support to 

 the coenosarc, and to prevent injurious flexion of the pliable 

 polypary 5 but it probably subserved other purposes of even 

 greater importance. No close parallel can be drawn between 

 the " radicle " of the Graptolitidse and the foot-stalk of the 

 Sertularidaj, since the former structure beyond doubt did not 

 serve as an organ of attachment. The central disk or cup of 

 some Dickograpsi, Tetragrapsi, and Dijjlograjjsi was compared 

 by Prof. Huxley with the basal plate of Defrancia^ a Bryo- 

 zoon ; but I think a more probable homologue is to be found in 

 the " float," or " pneumatocyst," of the Physophorid^, an 

 order of the oceanic Hydrozoa. The distal extension of the axis 

 is entirely without a parallel ; and when dilated, as in Diplo- 

 grapsus vesiculosus, Nich., it is difficult to conceive of any 

 function which it can have subserved. It cannot be of the 

 nature of a float, since it occupies the distal and not the proxi- 

 mal extremity of the organism ; and the most probable view 

 would perhaps be to consider it in some way connected with 

 the reproductive process. The second element, namely the 

 " common canal," is structurally a tube extending along more 

 or less of the axis, and giving origin to the cellules. Ordi- 

 narily it appears as a flattened space between the cellules and 

 the solid axis (PI. III. fig. 2); and it seems certainly to be a 

 distinct structure even in those forms in which the cell -parti- 

 tions are attached to the axis. As to the intimate nature of 

 the common canal, there seems to be no doubt that it conveyed 

 a soft connecting substance uniting the various polypites into 

 an organic whole ; and it is therefore homologous with the 

 " coenosarc " of the Hydrozoa. Its existence consequently 

 forms one of the strongest grounds for eliminating the Grap- 

 tolitidee from the Polyzoa, since no analogous structure is known 

 to occur in any of the latter. 



Of the cellules there is little to be said. They vary much 

 in shape ; but they usually constitute more or less cylindrical 

 or quadi-angular tubes, the bases of which are attached to the 

 common canal, whilst the opposite extremities terminate in 

 open mouths — the " cell-apertures." They thus come to re- 



