On the Distribution of the Rhabdophora. 185 



XXV. — On the Geological Distribution of the Rhabdophora. 

 By Chakles Lapwokth, F.G.S. &c. 



Part III. Results. 



[Continued from p. 29.] 



Family iv. Dichograptidse. 



This important group is at once the most prolific and the 

 most compact of all the families of the Graptolites. Although 

 it contains more than one third of all the recognized genera of 

 the Rhabdophora, no one can turn over the beautiful plates 

 which adorn Hall's classical memoir on the " Graptolites of 

 the Quebec Group," in which the majority of its forms are 

 figured, and fail to be struck with the decided family likeness 

 which pervades them all. The type of calycle remains sub- 

 stantially invariable throughout all its component genera ; and 

 I doubt not that the identity of this feature in the compound 

 family of the Phyllograptidas will eventually compel us to 

 regard it as also naturally belonging to the same subgroup ; 

 for its more fully known species are, morphologically, nothing 

 more than Tetragrapti whose branches, instead of remaining 

 free, are united dorsally throughout the whole of their extent. 

 So far as our present knowledge enables us to judge, it 

 appears that the Dichograptidaj constitute the most ancient 

 family of the Rhabdophora. The earliest examples appear 

 in the upper zones of the Lingula flags ; and the family in- 

 cludes all the Cambrian forms of Rhabdophora hitherto dis- 

 covered. Although but few species are yet quoted from these 

 ancient deposits, it is certain that many await discovery ; for 

 the family reaches its maximum, both in genera and species, 

 in the lower zones of the succeeding Arenig formation. In 

 the typical beds of this age, as exhibited in the strata of 

 Skiddaw, Scania, and Pt. Levis, all the genera, with but one 

 or two dubious exceptions, occur in association. In the 

 succeeding dark shales, which in Britain and Scandinavia are, 

 at present, provisionally assigned to the Upper Arenig and 

 Lower Llandeilo, few complex genera occur, and some species 

 of the simplest genus Didymograptus alone survive ; but 

 these are found in incredible multitudes. This genus is 

 represented by an occasional individual as late as the epoch 

 of the Upper Llandeilo (Glenkiln),< when the family appears 

 to have become wholly extinct. 



Didymograptus &c. — The simplest genus {Didymograptus) 

 appears to have been the most prolific of the family, and the 

 most extended in its vertical range. Its oldest known species 



