HAHN, DICTYONEMA-FAUNA OF NAVY ISLAND, N. B. 153 



in keeping distinct the lower level of Dictyonema flabelliforme forma 

 typica which is accompanied by Obolus salteri and appolinis and a large 

 Lingula f corrugata, and the higher zone of Bryograptus Icjerulfi, 

 which has associated with it Clonograptus tenellus, Dictyonema nor- 

 wegicum and Lingulella cf. lepis. It is Westergard who interpolates 

 between them a separate zone of Clonograptus tenellus and Bryograptus 

 hunnebergensis. My own observations harmonize with these statements 

 in a quite satisfactory manner, for I found that occasionally individuals 

 of all described species and varieties were met with on the same slab; 

 that, however, in one part of the collection studied, Dictyonema flabelli- 

 forme var. acadica and var. conferta prevailed, associated with Monobo- 

 lina refulgens, while, in another part, Dictyonema flabelliforme var. 

 ruedemanni was mixed with var. acadica, norwegica, desmograptoidea 

 and Lingulella ? cf. lepis, but that in both cases Staurograptus occurs 

 frequently. Although, of course, I am not able to make any statements 

 about the relative age of these two beds, the close conformity of the 

 facies on both sides of the Atlantic, when considered in a general man- 

 ner, seems to me to urge the assumption of homotaxial relationship, 

 provided we are willing to hold to the slight faunal differences due to 

 the geographical separation, for the supposed evolution of the Dictyo- 

 nema follows exactly in the same order as the historical succession. 

 Hence, the identification of Matthew's zones with the two lower beds of 

 Westergard seems, according to my opinion, to be correct. 



Significance of the Varietal Eange and its Phylogenetic Value 



Before balancing the observations on the Navy Island faunula, some 

 general questions bearing on the zoological point of view must be dis- 

 cussed. The first one has to deal with the term "variety," applied to the 

 observed changes of Dictyonema flabelliforme. The following points 

 come into consideration: The features in change are not only confined 

 to one or the other character of Dictyonema flabelliforme, but, as shown 

 by the diagram and textfigure 3, every feature of specific, even of generic 

 value is subjected to a more or less extended change. While a small 

 part of the observed varieties are mostly restricted to later stages of 

 Dictyonema flabelliforme, in some cases, e. g., var. ruedemanni, and 

 desmograptoidea, a considerable telescoping of the varietal features ap- 

 pears, which thus have already become inherent. Furthermore, not all 

 of these varietal features are interchangeable ; for instance, links between 

 var. ruedemanni and var. norwegica never occur. On those links, how- 

 ever, that are found, even new crossing features are observable (pyriform 



