440 Dr. Gerhard Holm — On the Structure of GraptoUies. 



This also points to conclusions drawn exclusively from the sicula 

 side. As above mentioned, Tornquist also has observed the thread- 

 like process sometimes arising from the apex of the sicula, and has 

 rightly indicated that the nearest correspondence to this in the 

 genera Diplograptus and Monograptus is the virgula. The two 

 species of Didymograptus which are here described, are both from the 

 grey glauconitic Vaginatenkalk at Halludden, near Torp in Boda 

 parish, in Oland. 



Didymograptus minutus, Tornq. Mut. PL XIII, Figs. 1-3. 



The three figures give, even at the first glance, a far clearer 

 picture of the external and internal structure of the proximal part 

 than any description. Fig. 1 shows a cast of the sicula side, with 

 the partitions appearing as dark lines on the rock surface ; Figs. 2 and 

 3 show the anti-sicula side in reflected and transmitted light, so that 

 the internal walls and growth-lines of the periderm become visible, 

 and also a simultaneous pictui-e of the external shape, the course of 

 the growth-lines, the internal divisions, and the connection between 

 all these thus obtained. 



The apex (initial part) of the sicula is broken off. All over the 

 remaining part, and the polypary besides, conspicuous and regular 

 lines of growth can be seen. The passage opening for the canal 

 arising from the sicula is in this case situated nearer the apertural 

 margin. This appears in Figs. 1 and 3 as a discontinuity in the wall 

 of the sicula. The canal arising from the sicula exhibits on the 

 anti-sicula side (Fig. 3) a peculiar characteristic and instructive 

 arrangement of growth-lines, showing how from the bud in the left 

 wall, by development in different directions, there is formed 

 simultaneously partly the "connecting" canal, which lies on the 

 doi'sal side of the sicula, and partly also the common canal of the 

 left stipe. Again, the left theca seems here to be completely 

 separated from the common canal by a wall which runs with the 

 sicula wall, and, without a break, stretches to the aperture of the 

 theca (interthecal wall between left theca and the succeeding theca). 

 This is confirmed by the lines of growth, which, on different sides 

 of the wall, do not stand in any relation to one another. 



On the sicula side (Fig. 1), on the contrary, matters are seen 

 from the cast to be quite different. The canal arising from the 

 sicula is here in free and open connection with the left theca, in 

 that the interthecal wall, mentioned on the anti-sicula side, takes its 

 origin at first nearer the middle of the left theca. From this it 

 follows that, simultaneously with the two above-mentioned canals, 

 the left theca is also developed from the bud on the sicula, but 

 that the development of the same proceeded from the sicula side, 

 where the connection exists. 



The interthecal wall between the left theca and succeeding theca 

 — the proximal part of the common canal is therefore formed falsely 

 (slceft) as well as obliquely — ends on the anti-sicula side, and opens 

 towards the common canal on the sicula side. The left theca, in 

 consequence of this, as shown by its outlines (Figs. 2 and 3), has 



