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
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