4 Sv. Leonh. Törnquist. 



tes; but, on the other hand, it is just as certain that it would be an unnatural 

 arrangement to unite such species as Rastrites Linneei Barr. or Bastrites maximus 

 Carr. with the typical Monograpti. And everyone who has made the graptolites 

 the subjects of closer studies must concur in Lapworth's reasoning that, if the 

 geuus Rastrites is to be suppressed because of the existence of the dubiously 

 ^transitional species Monographs triangulatus, »the great majority of admitted genera 

 must also be abolished on similar ground* 1 ). 



Essentially in accordance with Barrande's diagnosis, the genus Rastrites may 

 be characterized in the following terms: Rhabdosome uniserial, unbranching; com- 

 mon canal unusually narrow throughout, giving off isolated tubular thecse along its 

 whole extent. 



The delicacy of the common canal is a feature which should never be dis- 

 regarded, and whenever we find proximal graptolitic fragments showing isolated 

 thecse, there may, nevertheless, be reason to suspect that we have before us a 

 Monograptus, if the common canal is as wide as the thecse, or wider. 



Wim an in his valuable memoir »Ueber die Graptoliten» declares himself con- 

 strained to regard the true Rastrites theca as being represented by that part which 

 has previously been called a theca together with the internode next below 2 ). This 

 determination is quite consistant with his general conception of the thecee of the 

 other Graptoloidea. Having in another paper 3 ) set forth my position with respect 

 to this question, I shall, in this place, only remark, that a theca, as defined by 

 Wiman, coincides with that peridermal unit for which I have proposed the term 

 »thecal segment» 4 ). 



The characters by which the species of Rastrites may be distinguished are 

 afforded by (1) the general shape of the rhabdosome, (2) the form, size, and direc- 

 tion of the mature thecse, (3) the length of the intervals between them i. e. of the 

 internodes, (4) the development of these elements in the initial region of the 

 rhabdosome. 



The defining of the species by means of these distinctions, however, is at 

 times beset with considerable difficulties. Although the examples of some species 

 are often better preserved than might be expected from the tenuity of the inter- 

 nodes, it results from their fragility that, in many instances, only short broken 

 pieces are to be had, which permit no reliable conclusion concerning the original 

 form of the perfect fossil. Further, if we remember that the rhabdosomes of some 

 species assume their normal appearance at an early stage of growth, while in other 



') C. Lapwoeth, On Scottish Monograptidse; Geol. Mag. Decade II, Vol. Ill: p. 312. 

 2 ) C. Wiman, Über die Graptoliten; Bull, of the Geol. Instit. of Upsala, Vol. II, Part 2; 

 Upsala 1895; p. 31. 



s ) S. L. Toknquist, On the Diplograptidœ and Heteroprionidse of the Scanian Rastrites Beds; 

 K. Fysiografiska Sällskapets i Lund Handlingar; Ny följd, Bd. 8. (Acta Univ. Lundensis, Tom. 

 XXXIII); 1897; p. i. 



4 ) S. L. Töknquist, Researches into the Monograptidse of the Scanian Rastrites Beds; the 

 cited Transactions Bd 10 (Acta Univ. Lundensis, Tom. XXXV) 1899; p. 2. 



