NTCnOLSON SKTDDAW ORAPTOLTTES. 127 



c/rapsus and Didjimograpsus, which arc not known to transgress 

 the limits of the Lower Llundeilo rocks. Some of these, as JJiplo- 

 grapsus antennarms, Didymognipi^ufs V-fractus, &c., are exclusively 

 confined to the Skiddaw and Quebec groups ; whilst others, as 

 Didymograpsus geminus and D. hirundo, occur commonly in other 

 parts of the Lower Llandeilo series, and in other localities. In the 

 third section neither are the genera peculiar, nor are the species 

 exclusively of Lower-Llandeilo age ; hut this division includes forms 

 (such as Diplograpsiis terethiscidus, His.) which are of common 

 occurrence in the Upper Llandeilo rocks, and which seem to be ex- 

 clusively limited to the Upper portion of the Skiddaw series. 



In examining the species proper to the Slates, I shall briefly 

 consider the generic forms, with especial reference to those which 

 are peculiar to this horizon, and characteristic of it, since there 

 exists considerable divergence of opinion as to their classification 

 and nomenclature. 



II. Desckiption of the Genera and Species op Graptolites. 

 Genus Dichograpsus, Salter. 



This genus was proposed by Mr. Salter to include certain complex 

 Graptolites of the Skiddaw and Quebec series, the essential part of 

 the original definition being the possession of a '' frond repeatedly 

 dichotomous from a short basal stipe into 8, 16, 24, or more branches, 

 each with a single row of cells," the lower part of the stipes being 

 enveloped by a corneous cup (see Geologist, vol. i. p. 74, and Quart. 

 Journ. Geol. Soc. vol. xix. p. 139). Prof. Hall, however, in his 

 work on the Graptolites of the Quebec group, rejected the genus 

 entirely, and placed all the species belonging both to Dichograpsus 

 and to Teti'agrapsus in his utterly unwieldy genus Graptolithus. 

 The more complete materials now at our command have no doubt 

 shown that Mr. Salter was in error when he made the dichotomous 

 division, the even number of the branches, or the possession of a 

 corneous disk an essential point of the diagnosis ; for all these charac- 

 ters are absent in some Dicliograpsi : still the genus appears to be 

 both natural and convenient, and there seems to be no adequate 

 reason for its rejection. In point of fact, the reason which induced 

 Hall to include various branching forms in the genus GraptolitJius 

 was the mistaken belief that the simple monoprionidian Graptolites, 

 such as G. Sedgwickii, G. Sagittarius, G. priodon, (fee, had no real 

 existence in nature, but that they were always fragments of 

 compound species. The phenomena, hoAvever, observed in Britain 

 and on the Continent render it unquestionable that such simple 

 forms do absolutely exist ; and to these, therefore, the genus Grap- 

 tolithus should be in future restricted. At the same time it must be 

 freely admitted that in the absence of perfect specimens it is often 

 extremely difficult to settle the generic position of a species, and 

 that in some cases it may even be impossible. 



The characters of the genus Dichograpsus, as defined by our present 



