370 Graptolites. 



7. Dendrograptus, Hall. Polypary compound, with a 

 thick common stem, giving off branches irregularly, which 

 repeatedly sub-divide in a dichotomous manner. Two species 

 have been noticed in Britain, one from the Llandeilo and the 

 other from the Caradoc beds. Both species are founded on 

 fragments of the polypiferous branches, but these agree so 

 exactly with the species described by Hall, that there 

 can be little doubt as to the genus in which they should 

 be placed. 



Section II. — Species ivith two series of cells. 



8. Diplograpsus, M'Coy. Polypary having a slender, solid 

 axis, and with cells composed of true hydrothecaa. Nine 

 species are known, and all of them from the Llandeilo beds. 

 Fig. 2 represents one of the best known species, D. pristis, 

 His. sp. The proximal end is furnished with three spines, all 

 of which have the same origin ; the two lateral ones are not 

 the ornaments of individual hydrotheca3, but have the same 

 relation to the general polypary as the terminal one. Different 

 forms of these spines are figured at 2, a, h, and c. Fig. 5 is 

 ID. folium j His., which is destitute of any ornament at the 

 proximal end. The individual hydrotheca3 are marked by 

 parallel ridges, as if they increased in size, shown in the en- 

 larged portion at Fig. 5, b. Small specimens are figured at 

 5, c, and d. Fig. 4 represents an anomalous form, D. cometa, 

 Gein., having a very small number of cells (3 or 4) on either 

 side of the main axis, and these cells are very much produced, 

 so as to appear almost parallel to the solid axis. 



9. Climacograptus, Hall. Polypary, having a slender solid 

 axis, and with the cells hollowed out of the body of the polypary. 

 Three species are known in Britain, one from the Llandeilo beds, 

 another from the Caradoc, and the third common to both 

 series. Fig. 6 is 0. scalaris, Linn, sp., a species which has 

 been rejected by some authors' as only a state in which almost 

 any species might occur, and has been by others so misunder- 

 stood, that it has appeared under no less than ten different 

 specific names. The relation between specimens preserved so 

 as to show the cell mouths in profile (Fig. 6, b), and those 

 exhibiting them in a front view, as transverse or. " scalari- 

 form " markings on the upper surface, is beautifully shown 

 in a specimen figured (6, a), in which the polypary is so 

 twisted as to show the one set of markings on its upper half, 

 and the other on its lower half. I have noticed that the pro- 

 longed axis of this species at the proximal end is frequently 

 invested for a short distance by a sneath (Fig. 6, b). 



10. Retiolites, Barr. Polypary without a solid axis, cells 

 rising from a central common canal, and in contact throughout 



