HISTORY OF RESEARCH. cxxiii 



canal with many delicate tubes," were " still thinner in the Graptoloidea and less 

 likely to be preserved, so that they have almost entirely disappeared, and have only 

 exceptionally left behind traces of their existence." 



The structure of the Diplograpticlge is next discussed, and Diplograptus 

 uplandicus, n.sp., and Glimacograptus Kuckserianus, Holm, are taken as examples. 

 An interesting new form — Glimacog. retioloides — is also described. 



The family of the Phyllograptidas is retained by Wiman with full knowledge 

 of the demonstrations of Holm that its structure is essentially similar to that of 

 Tetragraptus. 



The histology of the Graptolite periderm is next discussed, and Wiman 

 concludes that in Monograptus there are only three layers, a middle thick one and 

 a thin one on each side of it. 



Retioluidea. — In the group of the Retioloidea Wiman gives a full description 

 of the structure of Retiolites nassa. 



Dendroidea. — The latter part of Wiman's paper is of especial interest, containing 

 many new facts of far-reaching importance connected with the group of the 

 Dendroidea. 



Wiman shows by means of sections that " in all Dendroidea there can be 

 distinguished three different kinds of individuals : nourishing individuals (which 

 he also calls thecse, since they doubtless correspond to the thecse in the Grapto- 

 loidea), budding individuals, and sexual individuals or gonangia." 



The Dendroid structure is described in great detail in the case of several 

 species and genera of the Dendroidea. In all cases the budding individual never 

 opens to the exterior, but itself gives rise to three new individuals, and these, as 

 they grow, gradually fill up the whole of the cavity of the mother budding theca. 



The species described include Dictyonema rarum, 1>. peltatum, D. tuberosum, 

 and D.flabelliforme. Dendrograptus (?) celandicus, D. (?) balticus, D. (?) bottnicus, 

 and Ptilograptus suecicus. 



The method of branching in the Dendroidea is carefully worked out in 

 Dendrog. (?) oelandicus. 



In Ptilograptus suecicus the structure is somewhat different. " The branches 

 carry twigs " which spring out alternately to right and left, and " these consist of 

 four individuals, opening one after the other." 



The mode of growth of these various forms of Dendroidea is very different, 

 some having a sicula, others having a disc from which a stem proceeds. In 

 Dictyonema peltatum " a large number of branches spread centrifugally within a 

 disc," and then rise up, " branch, anastomose, and join again by means of the 

 ordinary connecting fibres." " The proximal ends of the branches " in this species 

 do not " possess the intricate structure that characterises the distal parts," and 

 resemble those in a Monograptus. 



Wiman shows clearly from the foregoing that the older generic diagnoses of the 



