436 Br. Gerhard Holm — On the Structure of Graptolites. 



In tlie bilateral and diprionidian forms the sicula always occupies 

 a side position, more or less oblique to a median plane between the 

 second and third thecge, dividing the polypary symmetrically into 

 two similar halves. This arises in consequence of the fact that the 

 " connecting " canal between the second and third thecge, and thus 

 between the two side halves of the polypaiy, proceeds along one side 

 of the sicula, and more or less completely embraces it. The sicula 

 occupies, therefore, on one side of the polypary a more or less 

 superficial position, so that it is here exposed, either completely, as 

 in Didymogra'ptus, Tetragraptus, and Phyllograptus, or for a longer 

 or shorter space nearest the aperture, whilst the rest of it is 

 embedded in the polypary, as in Diplograptus and Climacograptus. 



This side of the polypary is called in the sequel the sicula side ; 

 the opposite side, which is occupied by the " connecting " canal 

 between the two halves of the polypary (second and third thecas) is 

 the anti-sicula side. 



The sicula here is either completely obscured — for example, in 

 Diplograptus, Climacograptus, and Phyllograptus, or visible only 

 near the aperture and mucro, as in such bilaterally developed 

 monoprionidian forms as J)idymograptus and Tetragraptus. In the 

 former the sicula on this side is completely embedded in the 

 polypary, and is concealed not only by the "connecting" canal 

 but also by the oldest theca and the common canal. In the latter, 

 again, the connecting canal forms only a broader or narrower band 

 across the sicula. 



The sicula side in descriptions is termed the anterior, that is the 

 side turned towards the spectator, which, when pointing out the 

 different position of the parts, is the usual position of the polypary. 

 For the same reason the anti-sicula side is termed posterior. In 

 this I agree with Tornquist — " Structure of Diprionidee " — who uses 

 the terms "obverse aspect" and "reverse aspect" in the same sense. 

 The second or left theca, as it may be called from its position, or 

 more correctly the bud from which it is afterwards formed, always 

 grows out from the left side, and lies along this near the aperture of 

 the sicula [Didymograptus and Tetragraptus) or ends against it, in 

 which latter case it makes a bend to the left {Diplograptus and 

 Phyllograptus). The apertures of the sicula and left theca are 

 always turned in opposite directions — that of the sicula to the right, 

 and in the same direction as the theca? of the rows or branches 

 lying on the right of the sicula; the left-hand theca towards the left, 

 and in the same direction as the remaining thecae on the left half of 

 the polypary. The direction of the boundary between the sicula and 

 left theca determines, therefore, the direction of the polypary. 



The position of the passage opening from the sicula, and thus the 

 place from which the bud grows out, varies. In Phyllograptus it is 

 situated quite close to the apex of the sicula, in Tetragraptus 

 Bigslyi, Hall, probably slightly lower down, in Didymograptus 

 minutus, Tornq., somewhat below the middle of the sicula, in 

 Didymograptus gracilis, Tornq. Mut., still nearer the aperture ; 

 but in Didymograptus gihberulus, Nich., the position is almost the 



