563.7 240 



II. 



The Structure of the Graptolites. 



II. — Retioloidea. 



THIS group is not yet well enough known for us to establish a 

 definite plan of structure, supposing such to have existed. In 

 any case, the group seems rather isolated from the rest of the 

 graptolites, as well as considerably more heterogeneous than the other 

 groups. Consequently a general account could not be given in a 

 small compass, but would involve detailed abstracts of several papers, 

 especially of those by Tullberg, Tornquist, Holm, and myself, together 

 with the reproduction of many figures. I therefore only reproduce 

 some figures from Holm (Figs. 10 and 11) and myself (Figs. 12-14). 



Fig. 10 shows a part of the net- work of Stomatograptus tornquisti, 

 Tullb., with the large pores and remains of the interthecal walls. 

 Similar interthecal walls have been observed by Holm in Retiolites 

 geinitzianus, Barr. Fig. 11 is taken from one of Holm's drawings of 

 the last-mentioned species, but has been simplified by the removal of 

 the whole net-work between the main strands : vv is called the 

 straight virgula ; zv the zigzag virgula ; pi the parietal strand 

 separating two thecae ; ml the mouth strand separating the mouths 

 of two adjoining thecae, also called the inner cross-rod. How the 

 proximal end was constructed, if there was anything like a sicula, 

 and, if so, how it was constructed, cannot be gathered from the 

 material to hand. 



In the work (7) from which these figures are taken, Holm 

 describes some fragments of a new species, which he refers temporarily 

 to the genus Retiolites under the name R. nassa. Of this species I 

 have succeeded in obtaining such good material that it has, perhaps, 

 become the best known representative for the group Retioloidea 

 (Fig. 12-14). Except for the complete strand and mesh system, and the 

 lower theca-edges developed into a kind of lid, nothing of the periderm 

 remains but here and there in the meshes small fragments of a 

 membrane, which probably among other things filled the meshes. 

 There is only one virgula, and it is straight. Morphologically it does 

 not correspond to the virgula in the Graptoloidea. A true sicula 

 cannot be distinguished, though there is an organ or individual 

 corresponding to it. This initial canal, as I at present call the part 

 from the proximal end to the first theca, is almost circular in section, 

 with a depression where the virgula is situated. A certain regularity 



