272 On some new Species of Oraptolites. 



D. caduceus cannot, therefore, be retained, though I have little 

 doubt that one of the forms subsequently figured by Mr. 

 Salter from the Skiddaw States under this name (Quart. 

 Journ. Geol. Soc. vol. xix. fig. 13 a) is identical with the 

 species now under consideration. 



That Didymograptus gibberulus is quite distinct from those 

 specimens which consist of two of the stipes of T. bryonoides 

 separated by the matrix from the other two, is at once seen 

 by a comparison of the cellules near the sicula in the two 

 cases. In T. bryonoides (as in all the Didymograpti except 

 the present species) the primary cellules are comparatively 

 small, the cellules not attaining their full development, or the 

 stipes their full width, till we have receded to some distance 

 from the sicula. There is thus a more or less extensive space 

 opposite the sicula, which is formed by the union of the bases 

 of the first pair of cellules and is narrower than the rest of the 

 polypary (PI. VII. fig. 5). On the other hand, in D. gib- 

 berulus the primary cellules are the most fully developed, and 

 the stipes are consequently broader in the neighbourhood of 

 the sicula than anywhere else (PI. VII. figs. 3, 3 a). 



There is also the very striking and unique peculiarity in D. 

 gibberulus that the two primary cellules do not diverge at an 

 angle from the top of the sicula, as is usual amongst the Didy- 

 mograpti, but are placed vertically, parallel with the long axis 

 of the sicula, and thus serve to divide the frond into two 

 halves (PI. VII. fig. 3 a). 



None of the specimens that I have seen exhibits the termi- 

 nations of the branches satisfactorily, the stipes in the largest 

 of them being about nine lines long. The sicula is generally 

 a broad blunt process, of a triangular shape, and about half a 

 line in length ; but it has occasionally a delicate thread-like 

 extension which raises its length to two lines (PI. VII. 

 fig. 3 b). 



In accordance with what we now know to be the true posi- 

 tion of the sicula and its relation to the development of the 

 celluliferous stipes, I have figured the specimens of this species 

 in what would, prior to Mr. Lapworth's researches on this 

 point, have been regarded as the reverse of their natural situa- 

 tion — namely, with the broader end of the sicula pointing 

 downwards. 



Locality and Formation. — Randal Crag and White Houses, 

 Skiddaw, Lower Skiddaw Slates. Collected by Mr. W. K. 

 Dover. 



