rw BRITISH GRAPTOLITES. 



1894. In a paper published during the same year Tornquist dis- 



'"''"7"' s/ ^ cussed some of the points in which he differs from Wiman as 



. ... ' T' to the structure of Blploqraptus. He controverts Wiman's 

 um Grraptohternas Ter- ± j ± 



minoloei " ' Geol statement that a " double longitudinal septum does not 



Foren. Forh.,' bd. .wi. exist," as in most species of Diplograptus it certainly is 

 present, either in a complete or incomplete form, " though in 

 no known case does it extend to the extreme proximal part of the polypary." 



He also discusses in detail Wiman's statement that there is no " common 

 canal," and that the " connecting canal " is only part of the first theca. According 

 to Wiman the whole rhabdosome consists of thecse only (excluding the virgula 

 and sicula), and Tornquist discusses whether this complete change in the nomen- 

 clature previously adopted is justified. 



Tornquist also urges that the question depends entirely on what significance is 

 attached to the word " theca," and he objects, in the third place, to Wiman's 

 statement that because the sicula gives rise to only one bud, therefore Diplograptus 

 is " monoprionidian." 



Two papers of first-rate importance, dealing with the 



tt 7 structure of the Graptolites, were published in 1895. 



" Oni Didymograptus, The first was by Holm, " On the Structure of Didymo- 



Tetragraptus, och graptus, Tetragraptus, and Phyllograptus." The important 



Phyllograptus," 'Geo!, results arrived at by him were due largely to the extreme 



Foren. Forh.,' bd. xvii. gkm with which he wag ab]e tQ ^ o]&te specimens from the 



matrix. 



The first conclusion arrived at is, that " in the main, a complete conformity 

 exists in the first stages of development of the polypary, both in the genera 

 Didymograptus, Tetragraptus, and Phyllograptus, and in the family of the 

 Diplograptidae." In all these the sicula gives rise to one bud only on one side 

 (left), and " from this bud is developed partly the second theca, partly the 

 ' connecting canal,' which connects both halves of the polypary, and which in the 

 first place gives origin to the third theca, and partly also the common canal, which 

 connects the second theca with the succeeding ones." In the Monograptidse, 

 which Holm regards as degenerate from the Diplograptidae, the " connecting- 

 canal " is entirely absent. 



He agrees with Wiman that the sicula consists of two parts, but he differs 

 in regarding the pointed end (initial part) as the original and oldest portion from 

 which the apertural part was developed. This apertural part, having the same 

 function as a theca, " might therefore be considered justly as the first theca of the 

 polypary." He uses throughout the terms first, second, and third theca, but holds 

 that the old name "sicula" is, nevertheless, a "convenient and significant one," 

 and should be retained. Referring to the common canal, he writes: "The 

 common canal, by which all the cells of the polypary are connected with one 



