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



1 1 



1899 Rastrites rastrum Eisel, Ueber der Zonenfolge ostthüring. und vogtl. Grapto- 



lithenschiefer, p. 6. 



Before entering upon the question of the original form of the species, it seems 

 appropriate to describe it such as it presents itself in fossil state. When most per- 

 fect, the rhabdosome commences with a sigmoid proximal portion, then continues 

 in a moderately arcuate division, and terminates in a more sharply curved distal 

 part (Figs. 1 and 2). The thecse arise in every part of the rhabdosome from its 

 convex side. As the rock in which the graptolite has been embedded is highly 

 affected by cleavage, their length as well as that of the internodes varies consider- 

 ably in different parts of one and the same specimen. Thus the thecse number 

 from 10 to 16 in 10 mm., but may originally have been 12 or 13 in that unit. 

 They have a length of 2.5 to 4 mm., and widen evenly from their bases to the 

 extremities, which are either emarginated or truncated, having an average width 

 of .4 mm. While in perfect specimens the thecse of the median division form 

 acute angles with the internodes next in advance, they are in shorter fragments 

 almost perpendicular to the common canal. 



When comparing examples in different state of preservation, one can scarcely 

 help the idea that the rhabdosome has originally formed a spiral line winding 

 round an imaginary low cone, and emitting perpendicular thecse just in the surface 

 of the same. If this be the case, the pressure of a superincumbent mass of rock, 

 combined with a subsequent lateral pressure, would bring about the very appear- 

 ances shown by the fossil examined. The delineation given by Richtek is by no 

 means incompatible with this assumption. 



The most conspicuous characters of this species are afforded by (1) the general 

 habit of the rhabdosome, (2) the form of the thecse, and especially of their apices. 



Localities and horizon. Thuringia: The specimens described have been 

 collected by Mr Eisel at Grobsdorf and Raitzhain in the zone of Cephalo- 

 graptus cometa (13). 



Rastrites phleoides Törnquist. 

 Pl. II, figs. 7, 8 (9, 10). 



1868 cfr. Graptolithus convolutus Carruthers, pars, Revision of the British Grapto- 



lites; Geol. Mag. Vol. V; p. 14, Pl. V, figs 1 b, 1 c. 



1887 Rastrites phleoides Törnquist, Anteckn. om de äldre paleozoiska leden i Ost- 

 thüringen och Voigtland; Geol. Foren, i Stockholm Förhandl. 

 Bd IX; p. 490, fig. 1. 



In the last-cited paper this Rastrites was characterized nearly in the following 

 way: The rhabdosome, so far as observed, forms a conical spiral line, the initial 

 part being, however, unknown; the thecse are 10 to 12 in 10 mm., and attain, in- 

 cluding their bristles, a length of about 5 mm., making with the common canal 



