312 C. Lapioortli — On Scottish Monograptidce. 



normal method of development. Variations of this nature are met 

 with in the species Monogra/ptus lohiferus, M'Coy, etc. In these 

 abnormal examples, which occur in abundance in some localities, the 

 sicula at the same time that it throws off the single polypiferous stem 

 in the usual manner, behaves also as if it were itself the primordial 

 hydrotheca of a second stem, which grows outwards in the opposite 

 direction to the normal one, and frequently rivals it in all its 

 characters and proportions. In this way the polypary becomes 

 actually bilateral. It is probable that all the Graptoloidea ought to 

 be regarded as colonies of siculas held together by a common body, 

 and on this view the occasional presence of this peculiarity can 

 hardly excite astonishment. We meet indeed with a parallel case in 

 Didymograpius, where the sicula seems to be the primordial and 

 common theca of two distinct monoprionidian series. 



§ II. — Genus Eastrites, Barrande. 



The genus Eastrites, founded by Barrande in his classical work 

 on the Graptolites of Bohemia, is most intimately allied to 

 Monograptus \Graptolithus) ; differing from it chiefly in the fact 

 that the hydrothecee, instead of being in contact or overlapping, as in 

 that genus, are invariably isolated. Its nearest allies are the forms 

 of the type of Monograptus SedgwicTcii, Portl. sp., in which the 

 hydro thecge just touch each other at their bases. In some examples 

 of that group, also, while the thecae of the distal portion of the 

 polypary are truly in contact, those near its proximal end are 

 isolated. This is notably the case in the form Monograptus triangu- 

 latus, Harkn. sp., in which it often happens that none of the 

 hydrothec83 are actually in contact ; and thus, broadly speaking, it 

 sometimes becomes a veritable Eastrites. The importance of this 

 fact was overestimated by Professor Geinitz, who concluded that it 

 was fatal to Barrande's view of the generic distinctness of Eastrites 

 and Monoprion ; and accordingly united them in his own new genus 

 Monograpsus} 



Richter is perhaps the only palaeontologist who distinctly coincides 

 with Geinitz in this opinion ; but some of our British observers 

 have shown indications of a tendency to follow in the same direction. 



It may at once be conceded that Monograptus triangidatus is the 

 connectiug link between the two groups ; and we may even go 

 farther and admit that there is much to be said in favour of the 

 theory that it was the direct ancestor of Eastrites peregrinus and its 

 allies. But if the genus Eastrites is to be suppressed because of the 

 existence of this dubiously transitional species, the great majority of 

 the admitted genera must also be abolished on similar grounds. In 

 seeking generic characters among fossils like the Graptolites, we are 

 perforce restricted to those afforded by the hard parts of their 

 external skeleton. No genus is more strongly individualized in this 

 respect than Eastrites. It is impossible to overlook the fact that all 

 its species are far more intimately allied inter se than they are to 

 any species of Monograpdus whatsoever. Thus, for the purposes of 



1 Die Graptolithen, s. 18. 



