Observations on the genus Rastrites and some allied species of Monograptus. 1« 



Note. The majority of species of the genus now dealt with which accompany 

 or succeed Rastrites peregrinus, maintain the general shape of the prototype, and 

 differ from one another chiefly in the development of the thecse and the internodes. 

 Before dying out, the genus presents us with some species in which the generic 

 characters appear, so to say, in an almost exaggerated form. 



Genus Monograptus Geinitz. 

 Monograptus triangulatus Harkness. 



For the synonymy, description, and illustration of the typical Monograptus 

 triangulatus, I refer to my paper » Researches into the Monograptidœ of the Scanian 

 Rastrites Beds», p. 19. As remarked by Lapworth *), specimens are occasionally 

 met with having all the theere seamingly isolated. In striking contrast with 

 such Rastrites-\\ke specimens is a form occurring in Thuringia, to which Eisel has 

 assigned a separate title. 



Monograptus triangulatus Harkness var. Raitzhainiensis Eisel. 



PI. Ill, figs 2—4. 



1899 Monograptus triangulatus Harkness var. Raitzhainiensis Eisel, Ueber die Zonen- 

 folge ostthüring. u. Vogtland. Graptolithenschiefer, p. 7. 



Without expressing a,\xy opinion of my own as to whether the peculiar 

 appearance of this form may justify the establishment of a distinct variety, or can 

 be explained as due to accidental deformation resulting from lateral pressure, I 

 give here some drawings of the graptolite, adopting, lor the present, the name 

 proposed by Eisel. The sicular portion, which very much resembles that of Hark- 

 ness' original figure of the type species, is strongly iucurved, often so as to form 

 a narrow loop, and produces isolated thecœ 2 ). The distal part is slightly arcuate, 

 or nearly straight. In this region the thecal are in contact with each other, but 

 broader and shorter than in the typical form, and terminate in more rounded 

 apices. Whichever may be the true nature of this form, its distal thecse differ 

 noticeably from those of any Rastrites, but show sometimes a superficial approach 

 to those of Monograptus fimJtriatus Nich. 



Localities and horizon. Thuringia: the specimens delineated have 

 been collected by Mr Eisel at Raitzhain; I have myself found similar examples at 

 Thierbach. The form occurs in the zones of Monograptus triangulatus and 

 Rastrites peregrinus (zones 12 a and 12 b), being more common in the former. 



') Lapworth, loc. cit. p. 312. 



2 ) The form of the initial part of Monograptus triangulatus seems to be unusually variable. 

 Lands Univ:s Årsskrift. N. P. Afd. 2. Bd 3. 3 



