354 J. Hopkinson — Structure of Graptoiites. 



Mr. Carruthers then described this group as a distinct genus, 

 restricting it to the typical species D. ramosus, and its allies ; and at 

 the same time, from an examination of British specimens, he dis- 

 sented from Hall's conclusion as to its structure, which he considered 

 to be exactly similar, in its proximal portion, to Diplograpsus} 



I have already expressed the opinion that "in D. ramosus, the 

 hydrothecee appear to have the same structure as in CUmacograptus"'^ 

 and I now intend to show, that, so far as can be judged from the 

 imperfect state of preservation in which our British specimens are 

 found, not only this species, but the entire genus, although frequently 

 somewhat similar in the form of its hydrothecse to Diplograpsus, is, 

 nevertheless, more nearly allied to Climacograptus. 



The genus may be thus briefly described : — 



Genus Bicranograptus, Hall. Grrapt. Queb. Gr. p. 112. (from 

 BiKpdvo^, two-headed ; jpd(j)Q}, I write.) 



Polypary with a double series of hydrothecse on the proximal 

 portion or stem, which divides distally into two branches, bearing 

 hydrothecae, continuous with those on the stem, on their outer 

 aspect only. 



The proximal portion is composed of two distinct series of hydro- 

 thec^, each having its own common canal, as shown by their 

 protecting periderms separating from each other in the axil of the 

 branches.^ From this point, the branches, bearing hydrothecse on 

 their outer aspect only, diverge symmetrically, forming various 

 angles with each other. The solid axis, commencing in a minute 

 radicle, ) which is flanked by two lateral spines, processes from the 

 first- formed thecaa,) runs up the centre of the polypary, between its 

 coalescing dorsal walls (?), and, being double, bifurcates in the axil of 

 the branches, becoming thus dorsal to each branch. The hydrothecse 

 are usually undistinguishable from each other for some portion of 

 their length ; we cannot trace the division between any two adjoining 

 hydrothecee towards their proximal end, nor can we say where the 

 periderm and its thec^ are connected : but sometimes a line of 

 separation can be traced, the distal margin of one theca having 

 parted from the proximal margin of the next. The apertures ex- 

 tend right across the distal end of each hydrotheca, from its apex to 

 its junction with the next succeeding theca. They are apparently 

 situated in a hollow, or incision, in the body of the polypary, and 

 are therefore not exposed beyond the genei'al margin of the polypary. 



It is evident that we can only compare the proximal, or diprioni- 

 dian portion of the polypary with the genera Diplograpsus and 

 Climacograptus, as they have a double series of hydrothecse through- 

 out their length. But let us first see in what theg difi"er from each 

 other. Hall gives sections of Climacograptus typicalis^ which show, — 

 first, that in this species the solid axis is a slender cylindrical rod or 



1 Geological Magazine, Vol. V., p. 132. 

 ^ Journ. Quekett Micros. Club, vol. i., p. 160. 



^ That the two series of hydrothecse are really thus isolated, I am by no means 

 certain. Pending' further investigation, I quote the view generally entertained. 

 * Grapt. Quebec Group, pi. A., fig. 4-8. 



