cxxii BRITISH GRAPTOLITES. 



He next discusses certain controversial points with refer- 



U7 /mr/ , ence to the work of Tornquist and Holm. He accepts the 



" Ueber die G-rapto- facts obtained by Tornquist, and considers that their diffe- 



liteu," ' Bull. Greol. rences of opinion concern questions of terminology only. 



Iustit. Upsala,' By the term ls theca » he meang « a part Q f tne test of 



a bilaterally symmetrical animal," and he repeats that " the 

 individuals corresponding to the thecae were developed from other similar indi- 

 viduals, and not from a substance contained in any common canal." " This," he 

 writes, " is not only clear from the course of the growth-lines," but receives 

 additional support from the recent discoveries made by himself as to the structure 

 of the Dendroidea, in which there is no common canal. 



He acknowledges the correctness of Tornquist's view that there is possibly 

 always a longitudinal septum in Diplograptus. 



He agrees generally with Holm as to the structure of Didymograptus, but 

 considers it advisable to retain the word " sicula," though he regards it as 

 probably the first theca, and he accepts Holm's view that the apical part of the 

 sicula is the initial part and the youngest. 



"Wiman then gives a classification of the Graptolitidae in general. This agrees 

 in essentials with that proposed by Lapworth (1873), but is modified in some 

 particulars. 



The family of the Monograptidas is first described, and the typical structure of 

 a Monograptus is exemplified in Monog. dubius, M. lobifer, and M. discus. 



He places the genus Azygograptus in the family of the Monograptidas on 

 account of its having only one row of cells, but he considers it to be a Didymo- 

 graptus -like form in which one branch is missing and that it probably belongs 

 to the Dichograptidae, with which it is also contemporaneous. 



Dimorphograptus may be considered as a transition form between the Diplo- 

 graptidas and the Monograptidas. 



The Dichograptidas he divides into two sub-groups according as they resemble 

 Didymograptus or Tetragraptus. 



He points out that in Didymograptus the opening between the sicula and 

 the first theca may not only occur on the initial or the apertural part, but may 

 occupy very different positions on the bilaterally symmetrical sicula. 



In the Graptoloidea he believes there is no "essential difference" between 

 monopodial and dichotomous branching, though in the Dendroidea it would have 

 more significance. 



Wiman attaches considerable importance to Hopkinson's discovery of partition 

 walls in Tetrag. serra between the common canal and the thecas, and he indicates 

 the analogy between this structure and that in the Dendroidea. He suggests that 

 the " Graptoloidea are only the most superficial periderm of the Dendroidea " : 

 " the proximal projections of the thecas in the Dendroidea, which fill the common 



