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



9 



unknown. The majority of fragments are more or less arcuate; some few being, 

 however, straight or even irregularly flexuous. The thecœ number 10 in 10 mm., 

 are, as a rule, 1 mm. long, and, as far as I have noticed, never exceed 1.5 mm. 

 in length. As to their direction, the angle which they form with the stem oscillates 

 between 60° and 90°, an amount of variation which seems to be due to occasional 

 causes acting at the embedding of the fossils. 



The chief distinctive character of the variety is the shortness of the thecse, 

 which I have found constant in all fragments. In spite of the deficiency of the 

 characters given, the form is at a glance distinguished from all its congeners. 



Locality and horizon. Thuringia: Fragments of this form occur in 

 abundance at Böhmsdorf in the zone of Cephalograptus cometa (zone 13), being 

 sometimes crowded on the surfaces of the shale in the same manner as the speci- 

 mens of Monograptus gregarius Lapw. are at some Scottish and Swedish localities. 

 For this reason I have departed from my general principle not to denominate new 

 forms on so unsatisfactory materials as those from which the above description 

 has been drawn up. 



Note. In his work »Etudes sur les Graptolites de Bohême» l ) Peener de- 

 scribes two graptolites, which he considers as varieties of Rastrites peregrinus Barr., 

 and designates by the names: A. Var. longispinus and B. Var. approximatifs. In 

 my opinion both deviate so much, not only from the typical Rastrites peregrinus, 

 but also from one another, that I do not hesitate to conceive them as separate 

 species. With Rastrites peregrinus they agree in the fact that the thecse and the 

 internodes already at an early stage of the development of the rhabdosome obtain 

 their constant appearance, but they differ from it in having a different general 

 form and markedly longer mature thecœ. In confining the species I follow Per- 

 ner's diagnoses. 



Rastrites approximatus Perner. 

 1899 Rastrites peregrinus Barr. B. var. approximatus Perner, Études sur les Grap- 

 tolites de Bohême, IILième Partie, Section a, p. 9, Pl. 13, 

 figs. 36—40, 42, 43. 

 After forming a full spiral revolution, or even more, the rhabdosome graduates 

 into a moderately arcuate distal division. According to Perner the thecœ are 

 2.5 mm. long, while the length of the internodes varies from .6 to .8 mm. Of the 

 type species I possess only a few imperfect Bohemian examples, but I think the 

 following form may appropriately be referred to it as a variety. 



Rastrites approximatus Perner, var. Geinitzi n. v. 



Pl. I, tigs. 32-41. 



1852 Monograptus peregrinus Geinitz, pars. Die Graptolithen, p. 45, Taf, V, figs. 

 _ 11 a > b ( ? U c , 12 )- 



x ) IILième Partie, Section a, p. 9. 



Lunds Univ:s Årsskrift. N. F. Afd. 2. Bd 3. 2 



