Dr. Gerhard Eohn — On the Structure of Graptolites. 489 



and immediately above, the common canals for both the right stipes 

 of the polypary. Again, from the left end of the connecting canal 

 arises only the common canal for the left stipes. 



The three primordial thecfe, representing the Didymographis-stB.gQ , 

 in Phyllograptus, occupy a somewhat oblique position in relation to 

 the diagonals, which in the cross-section of the polypary are parallel 

 with the thecal rows and describe a square : PI. XIV, Fig. 5. 



No trace of a virgula has been observed. The four wings in the 

 longitudinal septum are rarely completely regular, but the form of 

 the cross-section varies somewhat in longitudinal direction in the 

 same specimen, without any definite rule (PI. XIV, Fig. 8). Usually 

 the wings are united two and two to a connecting lamina (PI. XIV, 

 Figs. 5, 11, and 12). The winged laminee sometimes show a curved 

 wrinkling. As an abnormality, there was observed in one specimen, 

 on the anti-sicula side, a small supernumerary independent theca, 

 probably arising from the connecting canal. A deformity observed 

 in another specimen, was the presence of only three stipes. The 

 proximal end of this is not preserved, therefore I cannot determine 

 how the deficiency arose. 



EXPLANATION OF PLATE XIII. 



DiDYMOGKAPTUS MiNUTus, Tornq. Mut. 



Figs. 1-3. — Proximal part of the polypary, showing the sicula, broken off near the 

 apex, the left and right thecse, and the connecting canal between them ; 

 also the proximal part of one other theca on each branch. — Enlarged 

 38 diameters. 



Fig. 1. — Sicula side of the specimen, the rest of which is embedded in the rock; 

 the cast with the periderm nearly entirely removed in order to show the 

 walls, which appear as black lines, between the sicula on the one side and 

 the left and right thecal on the other side ; also the incompleteness of 

 the first-mentioned wall, by means of which the connection between the 

 sicula and left theca is effected. 



Fig. 2. — Anti-sicula side, isolated by means of acid, in reflected light, showing the 

 roll-shaped connecting canal and the different shape of the left theca on 

 this side to that on the sicula side, from which arises the inequality in the 

 sides of the interthecal wall. 



Fig. 3. — The same side, seen in transmitted light, so that the walls and the delicate 

 growth-lines of the periderm are visible, showing also the limits of the 

 thecse, by the intersecting of the growth-lines at different foci of the 

 microscope, and the connecting canal between the left and right thecse. 

 The wall of the canal next the right branch is somewhat broken, and 

 here in consequence only the growth-lines of the sicula are seen. 



Figs. 4-6. — An unknown Dichograptid, very young? specimen, consisting of only 

 the sicula and left theca and the connecting canal, one corner of which 

 projects beyond the sicula. On account of its small size and fragile 

 nature, the proximal part of the sicula was considerably injured before 

 the drawings were made. Slight fractures of the margins of the sicula 

 and left theca have been restored in the figures. — Enlarged 22 diameters. 



Fig. 4. — Sicula side, in direct light. 



Fig. 5. — Anti-sicula side, in direct light. 



Fig. 6. — Same side, in transmitted light, showing the growth-lines. The half of 

 the periderm belonging to sicula side is ground off. 



DiDYMOGRAPTUS GRACILIS, Tomq. Mut. 



Figs. 7-8. — Proximal part of a developed specimen. — Enlarged 10 diameters. 



Fig. 7. — Sicula side. 



Fig. 8. — Anti-sicula side, showing the connecting canal. 



