434 Br. Gerhard Holm — On the Structure of GraptoUtes. 



In this paper an account is given of the development and structure 

 of the polypary in the genera Dichjmograptus, Tetragraptus, and 

 Phyllograptus. This is based exclusively on the excellent material, 

 vs^hich I have succeeded in collecting during many years, from the 

 grey glauconitic Yaginatenkalk of various localities in N. Oland. 

 This limestone contains but few Graptolites, but they are in a con- 

 dition well fitted for isolation. 



According to the general current opinion, which seems to have 

 been founded on Lap worth's description of the development of the 

 graptolitic polypary in " On an Improved Classification of the 

 Ehabdophora," the development in the bilateral and the diprionidiaa 

 forms takes place from two buds, which arise directly on the 

 two sides of the sicula. Certainly Lapworth himself has since 

 changed his former opinion, and with his usual penetration, on the 

 ground of the structure of the genus Dimorphograptus, has stated as 

 theoretically more probable that the sicula in all the "Graptoloidea" 

 gives rise to only one bud : this opinion, however, has been 

 disregarded. Quite recently, first Tornquist and then almost 

 simultaneously Wiman, who had at their disposal far better material 

 than Lapworth, conclusively showed that in the Diplograptidae 

 only one bud is developed from the sicula, and that both rows of 

 thecse are derived from this. 



In the following pages it will be shown that, in the main, a com- 

 plete conformity exists in the first stages of development of the 

 polypary, both in the genera Didymograptus, Tetragraptus, and 

 PJiyllograptus and in the family Diplograptidae. It may, therefore, 

 be considered as in the highest degree probable that the earlier 

 development of the proximal part (the three first thecEe) in all 

 the bilateral or diprionidian forms of Graptolites is, in the main, 

 the same, and has taken place through the formation of only one 

 bud on one side of the sicula — or first theca, as I believe it is — 

 which side is always the same in relation to the later development 

 of the polypary. From this bud thereafter is developed partly the 

 second theca; partly the canal — "connecting canal" — which connects 

 both halves of the polypary and which in the first place gives 

 origin to the third theca ( = first theca on opposite side of sicula) ; 

 and partly also the common canal, which connects the second theca 

 with the succeeding ones. 



In the family of the Monograptidae, which must be considered as 

 degenerate from the Diplogi-aptidae, the " connecting " canal, which 

 otherwise bends over to the opposite side of the sicula, and from 

 which the thecal row of that side ought to come, is not developed, 

 but is entirely absent. From the bud on the sicula, therefore, only 

 the second theca is developed, together with the common canal for 

 the single or main branch of the polypary. From the sameness of 

 the structure of the proximal part arises the intimate relationship 

 between the Graptolite forms, separated in time from one another ; 

 and this also determines how the younger forms arise from the older 

 through a gradually increasing simplification at first of the distal 

 part, and then, as in the Monograptidae, of the proximal part of 

 the polypary. 



