1896. THE STRUCTURE OF THE GRAPTOLITES. 243 



At a certain distance from the proximal end the virgula leaves its 

 former place in the periderm net and runs free within it, so as to rejoin 

 the periderm in the middle of the distal spine. The periderm also 

 changes. In the region of the most distal thecae the main strands 

 begin to be effaced again ; the periderm often becomes somewhat 

 swollen out, and again assumes a somewhat circular shape. Almost 

 the only regularity distinguishable in this confusion of meshes is the 

 union of the strands in the distal spine, in such a manner that it seems 

 as though the virgula, which here meets them, branches dichoto- 

 mously four times, and in that way passes into the net-work. Further, 

 two pairs of branches of the fourth division include two triangular 

 meshes, which lie opposite each other in the effaced thecal fields, and 

 these triangles Holm has explained as the rudiments of the last thecas. 

 The interior cross-strands are not present here. 



As Holm has already pointed out, as regards the arrangement 

 of the thecae, this species bears the same relation to Retiolites geinit- 

 zianus as Climacograptus bears to Diplograptus. 



The material examined is from Lilla Carlso, a small island near 

 Gotland, and is said to come from bed c of Lindstrom, which 

 corresponds to the Wenlock Shale. 



HI. — Dendroidea. 



In 1890 Holm described in Dictyonema cevvicovne, Holm, some 

 appendages to the thecae, shaped like birds' nests, and considered that 

 they might possibly be gonangia. Subsequently, when I examined 

 Dendroidea, by means of series of sections, I discovered in the periderm 

 cavities situated just like these nest-shaped organs. In Fig. 15, 

 which represents Dictyonema ravum, Wiman, a little hole is to be seen 

 near every second theca. On the opposite side there are similar little 

 holes alternating with those on the visible side. It is these that 

 correspond to Holm's nest-shaped organs. 



We now proceed to Fig. 16, which represents selected sections 

 from a complete series of this graptolite, and compare the different 

 sections with Fig. 15. 



In all Dendroidea one can distinguish three different kinds of 

 individuals : noiirisJmig individuals, which I also call thecae, since they 

 doubtless correspond to the thecae in the Graptoloidea ; these are 

 denoted in Fig. 16 by t, t^, tj,, etc.; budding individuals, which are 

 denoted by ^1, ko, etc. ; and sexual individuals or gonangia, denoted by 

 g, gi, g-2, etc. 



In section i. Fig. 16, are seen three thecae t, t^, and the small 

 undenoted hole to the right of t^ between gi and k^ ; besides these 

 are seen two gonangia, g and gi, and finally two budding individuals, 

 one of which consists of the cavity in which the above-mentioned ^2, 

 gi and A'l lie, the other of k^. In section 2 the gonangium g, 

 which up till now has been on the left side, has pressed between t and 

 the other individuals, and opens on the right side. In section 3, t 



