476 NEW YORK STATE MUSEUM 



Tornquist [1881, 1892] first undertook the study of sections through 

 pyritized specimens, mostly diprionid forms, and observed the connection of 

 virgula and sicula, the position of the sicula on one side of the rhabdosome, 

 the presence of a "connecting canal," which connects the sicula and the 

 thecae, and the position of the virgula or solid axis within a median septum, 

 but his material did not furnish him any positive evidence in regard to the 

 single or double character of the common canal of the diprionid forms. 



A better insight into the structure of the graptolites was obtained by the 

 methods of dissolving graptolitiferous limestone, first applied by Gumhel 

 [1878], then used successfully by Holm and finally brought to considerable 

 perfection by Wiman} 



Holm elucidated, by means of such material, the structure of Retiolites 

 and Stomatograptus [1890], among the diprionid forms, and of Didymograptus, 

 Tetragraptus and Phyllograptus [1895] among the Dichograptidae. He 

 demonstrated the composition of the " f unicle " of thecae in the first two named 

 Dichograptidae, the perfect conformity in the development of the proximal 

 parts in all three genera and the fact that the frond of Phyllograptus is com- 

 posed of four stipes, coalesced at their dorsal sides. [For further details 

 of his results see the generic and specific descriptions of Phyllograptus, and 

 ch.lO]. The same distinguished author also first observed the presence of 

 smaller tubes attached laterally to the thecae in a species of Dictyonema, thus 

 giving the first intimation of the complex structure of the dictyonemas. 



Wiman [1893, 1895] demonstrated by his refined methods the bilaterally 

 symmetric form of the sicula and its composition of two parts, the initial part 

 of which is continued into a hollow rod, the nema ; he also showed the initial 

 distal growth of the first theca and its later reversion, and the origin of the 

 double row of thecae of Diplograptus and Climacograptus from one theca, 

 thus verifying Scharenberg's and Lapworth's assertion, that the so called 

 diprionid forms are also monoprionid. His important results on this subject 

 will be noticed more in detail hereafter. By the application of the microtome 



* See description, of these methods in ch. 3, p. 480. 



