Page:The Journal of geology (IA journalofgeology21894univers).pdf/282

Since in fact the graptolites are generally compressed and altered into a metallic sulphide, or are otherwise poorly preserved, and as this was also largely the case with the material examined by Lapworth, the genus Diplograptus was referred to the Diprionidæ, mainly by a comparison with other forms, particularly Didymograptus and Dicranograptus.

In 1876, Lapworth^[1] described two species of a new genus, Dimorphograptus, and, because of these, doubted the existence of any diprionidian forms. The proper view, he believed, was that the scicula in all graptolites develops but one bud. This view (an opinion founded on fact) that a monoprionidian scicula which at first gives off a monograptus-like hydrosoma could really give origin to a complete diplograptus-like distal end, has in later literature never been considered, but the older idea has been persistently retained to the present time.

The Scicula.In the present material, this is represented by 168 specimens, and of these 85 are separate. The form of the scicula is given in Pl. II., fig. 1-5 and 7-9. It is divisible into two essentially different parts, the distal one having a very thin and transparent wall, while the proximal is thicker and less transparent. Along the wall of the distal part are longitudinal thickenings or lines which branch and anastomose basally, and are lost near the boundary with the proximal portion. They unite, however, in the point of the scicula, and form the distal portion of the virgula to which I will again refer. Between the two parts of the scicula there is no septum.

In the proximal part of the scicula can be seen closely arranged diagonal lines, which I regard as growth lines. These have the same appearance as the often described thecal lines, differing only in the fact, that at a certain distance from the virgula, they gradually bend downward to join it at a sharp angle. In the very oldest part of the proximal portion of the scicula, the lines round regularly (Pl II., fig. 1), since the virgula, when these were forming, was not yet present. Very soon, however, they begin to exhibit a slight downward bending, and this increases