J. Hopkinson — On a New Genus of Graptolites. 21 



we suppose it is present in all. In one species, at the axil, the poly- 

 pary is slightly enlarged ; in others, a spine of variable length takes 

 the place of this enlargement ; while in another, the branches are 

 sometimes connected for a short distance by a membrane very like 

 the corneous disc of BicJiograpsus. The hydrothecse are undistin- 

 guishable from each other for the greater portion of their length, 

 from their apertures to their proximal end being intimately united 

 with the body of the polypary. Towards their distal end they are 

 for some distance free, their outer margins forming a continuous 

 curve ; curving outwards from their commencement, then more or 

 less convex, nearly parallel to the axis, and slightly produced as a 

 rounded lobe ; their apertures being situated in the rounded, pouch- 

 like cavity formed by this prolonged portion. In the several species 

 the hydrothecae differ more in the extent of their free portion and 

 the depth of the cavity, than in general form, except when the poly- 

 pary is twisted so that they appear in various positions on the shale 

 in which they are imbedded. Thus they are often preserved as 

 " scalariform impressions," or vnth their apertures appearing as 

 transverse oval markings on the exposed surface of the polypary. 

 It is when thus preserved that we realize most fully the intimate 

 alliance of this genus with Climacograptus. Except for the difference 

 in the width of the polypary, a fragment of a branch of Dicello- 

 grapsus, or of Dicranograptus, could not be distinguished from a 

 fragment of Climacograptus. 



There are one or two points in this description which demand 

 special attention. In the first place, the position of the radicle, and 

 its relation to the entire organism, is a controverted question. Dr. 

 Nicholson, in an elaborate paper " On the British Species of Bidymo- 

 grapsus," maintains that the spine I have mentioned as being 

 developed in the axil of the branches, is the true radicle, and conse- 

 quently he has been compelled to figure the species having this 

 spine in an inverted position, while those without it are correctly 

 figured.^ Mr. Carruthers considers that (in this genus) the theoreti- 

 cal position of the initial process is in the axil of the branches, 

 whether a spine is present or not,^ and gives figures of species with 

 and without this spine, in their correct position ; thus making his 

 " initial process " proceed from the side of the polypary which is 

 opposite its proximal or initial end.^ Whatever be the nature of the, 

 so-called, radicle, or initial process, it could not occupy such a 

 position, for in all Graptolites it unquestionably forms the proximal 

 termination of the organism, and in Dicranograptus, the most nearly 

 allied genus, and also in several species of other genera, it is flanked 

 by two lateral spines, just as it is in Dicellograpsus. The conclusion 

 is inevitable — this is the true radicle, and the axillary spine, which 

 is only occasionally present, whatever it may be, is, as far as we 

 know at present, an organ without its analogue in any other genus. 



This determination settles another disputed point. Dr. Nicholson 



1 Ann. and Mag. Nat. Hist., series iv., vol. v., p. 351. 



2 Mr. Carruthers has verbally informed me that the above is his opinion. 

 2 Geological Magazine, Vol. V., PI. v. 



