482 Dr. Gerhard Holm — On the Structure of GraptoUtes. 



of the left stipe. It follows from Moberg's description that the 

 relation is completely the same in Isograptus, as we shall see. 



According to Moberg, whose material consisted entirely of much 

 compressed specimens in shale, the impressions in the shale of the 

 two sides of the polypary present "as regards the proximal part" 

 (including the sicula) "'' a somewhat different appearance." On the 

 one side (dorsal side of Moberg) the " sicula," according to Moberg's 

 interpretation, would be divided longitudinally from the middle, 

 and would form "two similar primordial thecse grown together 

 along their whole length," except near the apertures. To each side 

 of the " sicula " a theca is attached, and grows with it for the whole 

 of its length. Again, on the opposite side of the polypary (ventral 

 side of Moberg) no such division in the proximal part of the sicula 

 has been observed, and the " sicula " passes up on the dorsal side 

 into two lateral thecse, between which the divided apertural part of 

 the sicula (primordial thecae) is visible. 



On comparing the figures and descriptions with those of D. minutus, 

 Mut., it is clear that figs. 3 and 7a represent the sicula side ; 

 to fig. 76 and the remaining figures there is no reference, but 

 fig. 5, and probably also fig. 4, represent the anti-sicula side with 

 the connecting canal, which Moberg regards as a longitudinally 

 divided sicula ("primordial thecse") consisting of the sicula and the 

 left theca, which latter, as in PhyUograptiis and probably also in 

 Tetragraptus, arises either at the apex of the sicula or immediately 

 near thereto. It is clear that the partition wall between the true 

 sicula and the left theca has not left any impression on the shale on 

 the anti-sicula side, from the fact that it is concealed by the con- 

 necting canal, which forms on this side the proximal part of the 

 primordial thecfe, and passes over on one side into the right theca, 

 and on the other into the theca immediately succeeding the left 

 theca. Although Bidymograptus gihberulus differs from J), minutus, 

 Mut., in the position on the sicula of the point of origin of the 

 left theca, and agrees in this respect with Phyllogrdptus angustifolius, 

 Hall, and Tetragraptus Bigshyi, Hall, this is not a sufficient 

 reason for retaining the genus Isograptus, for to judge from 

 Tornquist's figures (Siljansomradets Grapt., I, pi. i, figs. 13, 14) 

 the same would be the case in such a typical Didymograptus form 

 as D. decens, Tornq. In this connection a question concerning the 

 structure of 1). gihberulus may be discussed. In the generic diagnosis 

 and afterwards in the descriptions, ]\Ioberg characterizes Isograptus 

 without reservation as a two-stiped form. In spite of this he seems, 

 as far as I can understand, curiously enough, to contradict himself 

 at the end of his descriptions, and inclined to accept the existence of 

 four branches in the same, where, mentioning that " the apex of the 

 sicula ends in a narrow tubular appendage, also observed in very 

 young individuals, and that this appendage seems at a considerable 

 distance from the sicula gradually to increase not inconsiderably in 

 width," Moberg remarks — " Since this dilatation never takes place 

 much outside the part of the appendage enclosed by the stipe, we 

 should not run any risk in considering the appendage of the sicula 



