•ISO J)r. Gerhard Sohn — On the Structure of Graptoh'tes. 



In consequence of the thickness of the periderm, I have not been 

 fortunate enough to make it sufficient!}' transparent to show the 

 course of the growth-lines. In most cases the specimens have been 

 well preserved, with the exception of the apex of the sicula, which is 

 nearly always crushed. 



As I could not sacrifice my best preserved specimen for sectioniug, 

 I have not been able to distinguish the connecting opening between 

 the sicula and left theca. Probably it is situated near the initial 

 point, as in Fhi/Uograj?tns angustifolius (PI. XIY, Fig. 9). The 

 development of the left theca. the connecting canal, the right theca, 

 and the common canals, agrees in all essentials with that of 

 Didi/mograpfns i^Pl. XIV, Figs. lo-16\ These figures very clearly 

 represent the Didi/moLiraptus-stage of a young specimen, in which 

 the four branches of the second order have not yet begun to be 

 formed. 



The difi:erences which are found between the Did ymog rapt iis-stRge 

 of T. Bigsbiji and that of D. viinutiis, Mut., may depend on the diflerent 

 angle of divergence of the stipes, and thus might be termed specific. 

 A comparison between the latter and T. fruticosiis, Hall, in which 

 the angle of divergence is about the same, would certainly show 

 complete correspondence in detail. 



The foramen between the left theca and the connecting canal, with 

 the left common canal, is visible from both the sicula and auti-sicula 

 side (see Page-illustration, p. 4So ; Figs. 1 and 2 show examples). 

 The detailed explanation of the figures gives an exhaustive de- 

 scription of the structure of the proximal part. Here only may 

 be noticed the great width of the connecting canal in T. Bigshiji, 

 and how it forms along the sicula and left theca a raised pro- 

 jection on the anti-sicula side, in consequence of the situation of 

 the foramen near the proximal end of the sicula ; these are, there- 

 fore, here nearly completely concealed by, and embedded in, the 

 connecting canal, in the interior of which they stand out in relief 

 (see Page-illustration, Fig. 2). On the anti-sicula side, therefore, 

 only the proximal apex and a narrow border of the sicula, and the 

 left theca near its aperture, are visible : similarly, as in Didywo- 

 graptus the connecting canal is somewhat oblique in relation to the 

 long axis of the sicula. In consequence of a direct cleaving of the 

 common canal by a vertical wall, the formation of two branches 

 on each side of the connecting canal occurs, whereby there arise 

 the four stipes of Tetragraptus. 



ExpiA^'Aiioy or PAGE-nxrsrRAiiox (see p. 4S5) printed in the text. 

 Tetkagrapttts Bigsbti, Hall. 



Fig. 1. — Proximal part from the sicula side, ■witli the outer walls of the theca? on 

 this side removed so as to show the interior. The foramen between the 

 sicula and left theca is not visible ; that between the left theca and 

 connecting canal is, however, shown on the left side : the anterior left 

 branch is completely cut otit. and of the posterior branch only the base 

 remains, which arises from the common canal. On the right side, the 

 right aperture of the connecting canal is indicated by the light, which, 

 behind the sicula, through the foramen of the left theca and connecting 

 canal, jointly falls into the background of the passage between the latter 



