Prof. Nicholson 8f J. E. Man'—Phyhgenij of the Grapfolites. 537 



genera have descenderl 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. And, indeed, similar phenomena have been shown 

 to exist in other groups of organisms. Thus, in a paper on '• The 

 Bajocian of the xAIid - Cotteswolds," published in the Quarterly 

 Journal of the Geological Society for August 1895, Mr. Buckman 

 gives reasons for supposing that "genetic observations on Brachi- 

 opoda indicate that biplicate Terebratulce are, at any rate in the 

 Jurassic rocks, independent, or heterogenetic, homoeomorphous deri- 

 vatives of non-plicate forms. The method of nomenclature hitherto 

 adopted rather tends, however, to confuse these biplicate forms 



Group 9. 



■illiiiiiMiliiir""*""'""^'*'^ i> ^jMtM^a^^at^iiMtMtMmmt 



immmm 



TETRAGRAPTUS approximatus 



DiDYMOGRAPTUS nitidus 



too-ether, either as the same species, or as varieties of one another — 

 and they are often difficult to distinguish— and totally overlooks 

 their relationship to the non-plicate forms (from which they appear 

 easily separable), thus ignoring their genetic affinities altogether." 

 The same authority further remarks that " analogous cases of the 

 heterogenetic development of similar but entirely unrelated forms 

 are well known among Ammonites. Mojsisovics has called such 

 cases, between non-contemporaneous Ammonites, ' heterochronous 

 convero-ence.' " The conclusions which Buckman has reached as 

 reo-ards " species " of Brachiopods and Ammonites appear to us to 

 be applicable to " genera " of Graptolites. 



Following our inferences concerning the relative value of the 

 form of the hydrotheca^ and the amount of the angle of divergence 

 to their legitimate conclusion, we may observe that when the angle 

 of divergence reaches 860°, so that the stipes of the polypary 

 coalesce by the whole of the dorsal surface, as in the members of the 

 confessedly artificial families Diplograptidte and Phyllograptidae, the 

 true relationships of the members of these "families" to one another 



