J. Hopkinson — Structure of GraptoUtes. 355 



tube, passing through the centre of a canal which is common to the 

 two series of hydrothecse, — and secondly, that the hydrothecge, where 

 they are in contact, are formed simply by excavations in the body 

 of the polypary. Dr. Nicholson, on the other hand, states that in 

 CUmacograptus scdlaris the two series of hydrothecse are not organi- 

 cally connected with each other, their dorsal walls " coalescing to 

 form the median septum, along the centre of which runs the solid 

 axis."^ This is precisely the structure of Biplograpsus. We have 

 also numerous figures, evidently accurate, of C. scalaris, — I refer 

 more especially to Scharenberg's, '^ — which show that the hydro- 

 thecas, where usually in contact, are occasionally separated by a 

 slight interval, proving that they are really distinct from the body 

 of the polypary, and not simply formed by excavations in its sides. 

 This, also, is the structure of Biplograpsus. In what then do they 

 differ ? In CUmacograptus, as also in Dicranograptus, the separation 

 of the hydrothecse is only occasionally seen, and very seldom extends 

 to the common periderm. In Biplograpsus it is invariable ; the 

 hydrothecse, though adjoining each other for some portion of their 

 length, are entirely distinct and complete in themselves to their 

 junction with the periderm. 



The two genera also differ in the position of the apertures of their 

 hydrothecse. In Biplograpsus they are situated at the apex of the 

 free end of each theca, while in CUmacograptus they are situated in 

 a hollow which appears to be excavated out of the polypary. In 

 Dicranograptus they are situated in an exactly similar manner, the 

 outer margin of each hydrotheca being brought over the aperture of 

 the one which precedes it. 



It is thus seen that in the isolation of the two series of hydrotheca 

 the three genera do not differ, while Dicranograptus and CUmaco- 

 graptus alone are alike in the usual intimate connection, and only 

 occasional separation, of the hydrotheca near their origin, and in the 

 position of their apertures. Dicranograptus, however, differs from 

 CUmacograptus in its hydrothecse being usually, but by no means 

 always, more prolonged distally, in which respect alone it more 

 nearly approaches Biplograpsus ; but it is evident that this is only a 

 slight variation in form, attended by no corresponding change in 

 structure. We may therefore conclude that the genus is most nearly 

 allied to CUmacograptus, essentially differing from it in the dividing 

 of its polypary (its generic character) alone. 



It is also nearly allied to a section of the genus Bidymograpsus, 

 which I intend to describe on a future occasion. 



The genus Bicranograptus is exclusively Lower Silurian. It fi,rst 

 appears, represented by a single species only, B. ramosus, in the 

 Arenig of North Wales, and is then seen in the Llandeilo of Wales, 

 and of the south and south-west of Scotland, where it attains its 

 maximum, both in the number of species and of individuals. From 

 one locality, Moffat, Dumfriesshire, five species have been obtained 

 — all that are known to occur in Britain. One species, B. sextans, 



1 Quart. Journ. Geol. Soc, vol. xxiv., p. 628. 



2 Ueber Grapt., pi. ii., fig. 17-22. 



