HISTORY OF RESEARCH. cxxvii 



Bryograptus and Didymograptus which agree with them most closely in the character 

 of the thecas and the " angle of divergence." Group 1 contains Bryograptus 

 Gallavei, Tetrag. ITieksii, and Didymog. affinis ; Group 3 Bryog. ramosus, Tetrag. 

 fruticosus and Didymog. Murchisoni; Group 6 Tetrag. Bigsbyi, Didymog. gibberulus, 

 and an unknown Dichograptus, and so on. In those cases where, in any corre- 

 sponding place in a given group, there is no species with the required characters to 

 fill the gap, the authors confidently assert that further research will probably 

 reveal its existence. The authors hold that the members of each of these groups 

 are phylogenetically related, and that it is very difficult to understand how the 

 " extraordinary resemblances between the various species of Tetragraptus and 

 Didymograptus have arisen, if, as usually supposed, all the species of these genera 

 have descended from a common ancestral form for each genus, in the one case 

 four-branched, in the other case two-branched;" "on the other hand, it is 

 comparatively easy to explain the more or less simultaneous existence of forms 

 possessing the same number of stipes, but otherwise only distantly related, if we 

 imagine them to be the result of the variation of a number of different ancestral 

 types along similar lines." They suggest that the genus Monograptus also may 

 contain " descendants of more than one family." 



The authors point out that if their conclusions are correct, the present nomen- 

 clature would have to be eventually altered. Meanwhile they propose to retain such 

 names as Monograptus, Didymograptus, etc., as " generic " names, but the " species 

 placed under these different groups do not belong to definite genera " (in the strict 

 biological sense of the word) : they constitute cases of what Buckman terms the 

 " hetero-genetic homceomorphy " of forms which are only distantly allied to one 

 another. 



They adduce briefly reasons for this " special case of mimicry, and endorse 

 Clement Reid's suggestion that the variations in form may be connected with the 

 supply of food " ; the necessity of providing food brought about a reduction in 

 the number of stipes, and also a change in the direction of these stipes. Those 

 series of hydrothecae which were farthest apart would have a better chance of 

 obtaining food, and thus the " angle of divergence " increased from a very small 

 angle until it reached its maximum of 360° in Phyllograptus, Diplograptus, etc. 

 Variations in the form of the hydrothecge may also be explained on the same ground. 



In a note to this paper, Nicholson and Marr suggest the new specific name of 

 Tetragraptus inosculans for those forms which resemble Tetrag. Bigsbyi, but in 

 which the stipes are in contact or even more or less fused. 



1895. 



Hall T. S. Hall discussed at considerable length the question 



" Notes on Didymo- of the synonymy of Didymograptus caduceus in a paper 



graptus caduceus, with published in 1895. He concludes that Salter's name D. 



remarks on its caduceus has priority over D. gibberulus, and therefore the 



synonymy," ' Proc. Eoy. ^^ ^^ f ^ Qut of ^ 

 Soc. Victoria,' vol. viii. 



