Dr. Gerhard Holm — On the Structure of GraptoHtes. 491 



Fig. 4. — The left side, seen a little from the left. The aperture of the lowest of 

 the left thecfe turned towards the spectator ; above, to the right, the 

 anterior, to the left the posterior, left thecal series. On the left side one 

 gets a glimpse behind the left theca of the apertural part of the sicula, 

 and behind the posterior left thecal series of the apices of the posterior 

 right thecal series. 



Fig. 5. — From the proximal end, with the sicula side turned upwards, showing in 

 the middle the aperture and the oblique position of the sicula, and on 

 each side of this the apertures of the left and right thecse. The apertures 

 of these three thecae occupy almost one and the same plane, which forms 

 an angle of about 40° with the cruciform plane of the thecal series. In 

 this figure all the thecae visible in the specimen in this position are 

 drawn. — Enlarged 6 diameters. 



Fig. 6. — Specimen with the anti-sicula side perfect, seen straight from this side, 

 so that only the two posterior thecal series are visible. The sicula and 

 the left and right thecse appear in the same position as in Fig. 3. 

 Isolated with acid. — Enlarged 4 diameters. 



Fig. 7. — The interior of another incomplete specimen, ground down directly to the 

 level of two thecal series lying in the same plane, and afterwards isolated 

 with acid. It shows the smooth interthecal walls and their straight 

 terminations against the common canal, and also the longitudinal septa. 

 Both the laminae of the longitudinal septum, which bounds the common 

 canal for the downturned thecal series, which are here not visible, are 

 complete, whilst of the two others only a fragment remains. — Enlarged 

 6 diameters. 



Fig. 8. — Impression of the outer side of two thecal rows lying in the same plane (to 

 the left the anterior right, to the right the posterior left) . Between 

 these projects the common canal formed by the longitudinal septa for 

 the thecal series, which are bent downwards and concealed in the rock 

 matrix. The longitudinal septa appear here, as is often the case, to be 

 somewhat irregular, and of a different shape in the longitudinal 

 direction. Transverse sections of the longitudinal septa, therefore, do not 

 maintain the same form throughout the whole length of the polypary. 

 At the proximal end, one side of the sicula embedded in the polypary is 

 free, and its oblique position in relation to the long axis of the polypary 

 is shown. To the right of the sicula the longitudinal septum of the 

 anti-sicula side is perforated by the connecting canal. — Enlarged 4 

 diameters. 



Fig. 9. — Specimen almost completely embedded in the rock, only a small part of the 

 proximal end of the sicula side being free, showing the conical proximal 

 part of the sicula and left theca, the greatest part freed from periderm, so 

 that the partition wall, as well as the connection between the two near the 

 apex, stands out. — Enlarged 6 diameters. 



Fig. 10. — A part of a specimen lying in the rock in the same position as the specimen 

 Fig. 8, showing the impression of the proximal part of the anterior right 

 thecal series, and the proximal part of the posterior left series ia relief. 

 The substance infilling the sicula has fallen away, except near the apex. 

 To the right of the sicula a trace of the conical elevated part of the left 

 theca, merging into the sicula near the apex. — Enlarged 6 diameters. 



Figs. 11, 12. — Transverse section of two different specimens, isolated with acid, 

 showing the commonest forms of the longitudinal septa, the common 

 canal for each thecal series, and the straight inner edge of the interthecal 

 walls. — Enlarged 6 diameters. 



Tetrageaptus Bigsbyi, Hall. 

 Figs. 13-16. — A quite uninjured young specimen, representing the Bidymograptus- 

 stage, showing only the sicula, left and right thecae, and the connecting 

 canal between them, and at both ends of this its passage into the 

 common canal, which at this stage has the appearance of, and may have 

 functioned as, the distal theca. As in nearly all my specimens of Tetra- 

 graptus, the apex of „the sicula and left theca is somewhat shrivelled. 

 Isolated with acid. — Oland, the cliffs south of Byrum's Sandbay, in the 

 glauconitic grey Vaginatenkalk. Enlarged 12 diameters. 



