Dr. Gerhard Holm — On the Structure of OraptoUtes. 437 



game as ia Phyllograptiis, if one disregards the long thread-like 

 initial part of the sicula. 



The connecting canal. — From the bud developed on the sicnla, 

 there arises almost simultaneously with the left theca the common 

 canal for the left half of the polypary and the connecting canal, 

 which crosses the dorsal side of the sicula and gives origin to 

 the third (or, better, the right) theca lying on the right side of the 

 polypary, and also the common canal for the right half of the 

 polypary. This is particularly clear and conspicuous in D. minutm, 

 Tornq. Mut., and the same thing also occurs in Tetragraptns and 

 Fhyllograptus (see Figures). By the connecting canal, therefore, 

 we understand in the sequel the canal surrounded on the outside 

 by a special wall, which crosses the sicula on the dorsal side of 

 the polypary, and forms the connection between the two halves 

 of it. 



By the development of the three thecse just described, namely the 

 sicula, the left and right thec£e (primordial thecal), there originates 

 the primitive form, from which by bifurcation and growth in diiferent 

 directions a further increase of the polypary takes place. This is, 

 in its general characters, in complete agreement in the representative 

 Graptolites referred to, and therefore probably similar in all 

 Graptolites where degeneracy has not taken place. 



The genus Didymograptus, from the left theca of which there 

 arises only one left branch, and similarly from the right only one right 

 branch, constitutes a further stage of growth of the primitive form. 

 This primitive form or state of development we can therefore term 

 the Didymograptus-stsige. If no other facts of a different nature are 

 known, it is impossible to assign an individual to its proper genus 

 when it is only known in this or in an earlier stage of development. 

 This is shown in Figures 13-16, PI. XIV, which represents 

 a young form of Tetragraptns Bigsbyi, Hall, in the Didyraograpius- 

 stage. An account of the further bifurcation belongs to the 

 description of the different genera. 



The virgida. — The virgula is wanting in the genera here described ; 

 it has, moreover, never been observed by me in any forms belonging 

 to the family DichograptidEe. On grounds which will be shown 

 later on, a virgula corresponding to that in JDiplograptus and 

 Monograptus cannot occur in the DichograptidEe. The same is the 

 case in the Dictyograptidee, Tullb., in forms of Nemagraptidte, and 

 in the genus Dicellograptus of family Dicranograptidee. A virgula 

 seems never to have been described or figured in any allied form. 

 In the family Diplograptidee a virgula has been observed in a great 

 many cases, and probably therefore occurs generally (or always?). 

 In the family Monograptidae it at least is often found, and in 

 many cases has been figured. I have not yet had an opportunity of 

 observing whether both the chitinous threads, which in the Eetio- 

 litidse {Retiolites and Stomatograptus) have been called the straight 

 and zigzag virgulas, correspond with and have the same mode of 

 origin as the virgula in Diplograptus. That a virgula corresponding 

 to that in Diplograptus and Monograptus cannot occur in the families 



