THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE IV. VOL. II. 



No. XI.— NOVEMBER, 1895. 



OK,IC3-IIsr.A.IL. JLI^TICLES. 



I. — On Dwymograptus, Tetkagraptus, and Pbyllograptus. 



By Dr. Gerhard Holm, 

 Palaeontologist to the Geological Survey of Sweden, Stockholm. 

 Translated from the original in Geol. Foren. i Stockholm Forhandl Bd XVII 

 Haft iii, No. 164, 1895, pp. 319-359. By G. L. Elles and E. M. R.'Wood,' 

 Newnham College, Cambridge. 



(PLATES XIII AND XIV.) 

 {Continued from the October Number, page 441.) 

 DiDYMOGRAPTXJS GIBBERULUS, Nicll. 

 Moberg, Nya Grapt. fr§,n Sk§,ne, p. 345, pi. viii, figs. 3-7. 

 S has been previously indicated (see p. 439'), the conclusion might 

 be drawn from Moberg's description and figures of this species, 

 interpreted by the knowledge of the true structure of the Bidymo- 

 graptus polypary, that the structure of the proximal part could in no 

 essential particulars be separated from that just described in D. mi- 

 rmtvs, Mut. Consequently the genus Inograptns, which was proposed 

 by Moberg for the species in question, is not authorized — at least, as 

 founded on any of the characters cited by Moberg. The differences 

 between Isograptus and Didymograptus, to which latter genus the 

 species is referred by Nicholson, ought to be, according to Moberg, 

 that in Didymograptus " both stipes arise at somewhat different 

 levels on the sicula," and that " each branch is itself bilaterally 

 symmetrical " ; whilst, on the contrary, in Isograptus the stipes should 

 "arise bilaterally symmetrical from the sioula," and each branch 

 is not itself bilaterally symmetrical. As we have seen above, in 

 reality in Didymograptus the left stipe is not bilaterall}'^ sym- 

 metrical, and. the symmetry in the right stipe is entirely the 

 same as that which is described by Moberg in Isograptus, namely, 

 obliquely near the base, because the stipes do not arise from the 

 side of the sicula, but from the "connecting" canal. 



No one believes for a moment that the presence of symmetry in 

 stipes is a distinction. The same is also the case as regards the 

 point of origin of the stipes. As has been shown above, the stipe 

 arises in Didymograptus from a point on the left side of the sicula, 

 and this is, therefore, the apparent point of origin of the right stipe 

 from the right side of the sicula, as is believed by Moberg, and 

 which, as has been previously shown, lies at a difierent level to that 



1 The reference in the October Number, p. 439, to PL XIV is an error ; it 

 should be to PL XIII. — Edit. Geol: Mag. 



DECADE IV. VOL. II. — NO. XI. 31 



