lxxviii BRITISH GRAPTOLITES. 



lg „ 2 The year 1872 was also marked by the appearance of 



Allman, Allraan's classical monograph on the ' Gymnoblastic or 



' A Monograph of Tubularian Hydroids,' in which a considerable section is 



the Gymnoblastic devoted to the discussion of the affinities of the Graptolites. 



or lubulanan Accepting without criticism the views already current among 



palaeontologists as to the structure of these fossils, Allman 

 draws from them some highly original and suggestive conclusions as to the 

 homologies of the various organs. He considers that it is doubtful whether such 

 anomalous forms as lietiolifes and Phyllograjptus should be included among the 

 Graptolitidas, while forms like Gorj/noides, Dendrograptus, and especially Didyonema, 

 are " almost certainly not Graptolites." 



Acknowledging that the resemblance of the polypary in the Graptolites 

 to the trophosome of a calyptoblastic Hydroid — Sertularian or Plumularian — is 

 " sufficiently obvious," Allman considers that while their affinities with the Hydroida 

 " are too decided to justify their omission from any complete exposition of the 

 palasontological history of this group of the animal kingdom," yet their peculiar 

 characters " necessitate the establishment for them of a separate sub-order of 

 Hydroida." For this he proposes the name Rhabdophora (Rhabdos = rod), from the 

 presence of the characteristic solid axis or virgula. The Polyzoan affinities of 

 the Graptolites are very briefly discussed, but he admits that " were it not for the 

 discovery of the graptolite gonosome, we should have nearly as much to say for 

 this view as for that which would refer them to the Hydroida." 



Allman discusses at considerable length the homologies of the most charac- 

 teristic structure of the Graptolite, viz. the virgula or axis, "the presence of which 

 can hardly be regarded as offering an insurmountable obstacle to the admission of 

 the Graptolites into immediate relation with the Hydroida." He regards it, like 

 the perisarc, as "an excretion from the ccenosarc." The distal and probably the 

 proximal prolongation of a naked axis beyond the celluliferous part of the poly- 

 pary he considers to be probably only an apparent phenomenon. He says that 

 there is "reason to believe that the coenosarc invested by a proper perisarc was 

 originally continued " along the rod, but this perisarc on account of its delicacy 

 has not been preserved. 



Denticles. — Perhaps the most original part of this work is that in which 

 All man suggests that the structitres in the living Hydroida homologous with the 

 denticles of the Graptolites are not the hydrothecas but the nematophores, such as 

 those of Aglaophenia, which contain simple protoplasm and not true liydranths. 

 lie points out that it is not only in general form that the nematophores resemble the 

 (iraptolite-eells, but also in their method of communication with the common canal, 

 for the continuous and open communication of the calycles of Graptolites with 

 the mail] tube is very differenl from the constricted communication (often associated 

 with an imperfect diaphragm) which exists between the hydrothecie and the 



