268 THE JOURNAL OF GEOLOGY. 



Since in fact the graptolites are generally compressed and 

 altered into a metallic sulphide, or are otherwise poorly pre- 

 served, and as this was also largely the case with the material 

 examined by Lapworth, the genus Diplograptus was referred to 

 the DiprionidcB, mainly by a comparison with other forms, partic- 

 ularly Didymograptus and Dicranograptiis. 



In 1876, Lapworth' described two species of a new genus, 

 Dimorpliograptus, 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 Scicida. In the present material, this is represented by 

 168 specimens, and of these 85 are separate. The form of the 

 scicula is given in PI. 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 vir- 

 gula, 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 (PI. II., fig. i), 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 



' On Scottish MonograptidcE. Geol. Mag., Decade II, Vol. Ill, p. 544, 1876. 



