Observations on the genus Rastrites and some allied species of Monograptus. 13 



interspaces increase rather rapidly until, at a certain distance from the sicula, both 

 attain a length of 3 mm. From this point the appearance of the rhabdosome 

 remains constant. The thecse form, as a rule, right angles with the stem, with 

 the exception of the oldest ones, which often have a deviating direction. 



The close relation of this form to the type species is evident. The initial 

 part of the latter has never been described, but there is every reason for believing 

 that it agrees with the corresponding part of the variety. In such case, the chief 

 distinctive character of this consists in the fact that the increase of thecse and 

 internodes ceases at an earlier stage of the growth of the rhabdosome than in 

 Rastrites distans, in which the said elements, according to Lapworth, attain a 

 length of about 5 mm. 



The question might be raised whether the variety might not prove to be 

 founded upon proximal fragments of the type species, but the constant size of its 

 constituting factors along fairly long fractions of the rhabdosome lends no support 

 to such an interpretation. 



As Lapworth has remarked (loc. cit.), Rastrites distans v. abbreviates displays 

 a superficial resemblance to Rastrites peregrines v. hybridns, which, however, obtains 

 its normal size at a still earlier age, and, moreover, differs by reclined thecse also 

 in the mature rhabdosome. 



Geinitz, in his last memoir claims Rastrites peregrinus v. hybridus Lapw., 

 Rastrites distans Lapw., and Rastrites maxirmis Carr. as synonymous with Rastri- 

 tes (» Monograptus ») Linncei Barr. 1 ). This conception is probably based on the 

 author's opinion that the different elements of graptolites have continued growing 

 for some longer time after their first development and, therefore, successively pre- 

 sented various aspects. 



Perner, again, in «Etudes sur les Graptolites de Bohême» regards Rastrites 

 distans v. abbreviatus as a »stade moyen de croissance» of Rastrites Linnœi Barr. 3 ). 

 If I am right in referring the initial fragments represented in figs. 11, 13 and 14 

 to the former variety, this deviates palpably from Rastrites Linnœi as described 

 and figured by Perner. 



Localities and horizon. Thuringia: Rastrites distans v. abbreviatus is a 

 common fossil in the zone of Rastrites Linncei (zone 14). I have collected speci- 

 mens at Grobsdorf, Rückersdorf, and Heinrichsruhe, and have received 

 from Mr Eisel specimens from Grobsdorf and Al t m an n sgr ü n; Sweden: the 

 fragments delineated in fig. 18 are seen in a boulder from En ån (Dalarne), and 

 those represented in figs. 19 and 20 were found at Tomarp (Skåne) in the zone 

 of Cephalograptus cometa; Bohemia: some imperfect graptolites collected by me 

 at Litohlaw seem to be referable either to the typical species or to this variety. 



') Loc. cit., p. 22. 

 2 ) Loc. cit., p. 8. 



