I916. No. 10. THE LOWER CAMBRIAN HOl.MIA FAUNA. 85 



development with the Mcrostoiiiata, as Dollo represented for us in his 

 epoch-making work "La paléontologie ethologique" '. 



We now pass over to Walcott's second line of development within the 

 Mesouacidae family, represented by the genera Callavia, Holmia\ ]Vaunena, 

 and in addition the new genus established by the present author, viz. 

 Kjcriilfia. According to Walcott these forms continued as the Middle 

 Cambrian Paradoxidae. 



I quite agree that these forms represent a group apart and different 

 from the Olenellns line. They exhibit certain points of agreement in the 

 general form of the body and in certain structural features, e. g. the 

 markedly developed and broad hypostome attachments. On the other hand 

 they differ from each other in several respects without it being possible to 

 understand whether these divergences are arranged in progressive or in 

 regressive series. This is readil}- shown by a further consideration of the 

 subject. The structural features the differentiation of which may be most 

 readily traced are: 



1. The form and structure of the glabella. 



2. The intergenal spines. 



3. The number of the thoracic segments. 



4. The form of the pleurae. 



The glabella oï Callavia is obviously the most primitive in structure. In 

 this genus it is narrow, with either almost parallel sides or sides converg- 

 ing anteriorly. The frontal lobe is small, often triangular, and the hinder 

 lobes are uniformly developed. On the whole it is a form of glabella that 

 strongly reminds one of the oldest and most primitive genus Xevadia. 



The glabella in the case of Kjcmlfia is somewhat more differentiated, 

 the frontal lobe being larger than in the preceding genus and the 2nd lobe 

 prolonged. 



We find further developments of the glabella both in Holmia and 

 \Va)inena\ the frontal lobe has become larger and broadly rounded, whilst 

 the 2nd lobe (especially in the case of Hobiiia) is remarkably geniculated. 



We find some of these points of differentiation in various young stages 

 of Holuiia (page 66). 



If we draw conclusions from the evolution of these genera we can 

 make the line: Callavia, Kjcruljia, IVauiicria, Holmia. 



Considering the in tergen al spines, that undoubtedly are a primary 

 and primitive feature, we find them most strongly and primitively developed 

 in Holmia in which the intergenal rib is distinctly preserved in adult speci- 



1 FiuLL. de la Soc. Beige de Geologie, Mémoires. To. XXIII, 1910. 



