1916. No. 10. THE LOWER CAMBRIAN HOLMIA FAUNA. 83 



In the case of Mesonacidac the rostrum, or the hypostome attachment 

 as we may term it with Moberg, has obtained its remarkable form by the 

 connecting sutures that is not as usual convergent with the hypostome, but 

 has a markedly divergent course, by which this part of the shell has be- 

 come long and narrow, and extends almost as far back as to the posterior 

 margin of the cranidium. 



This is probably a primitive state, and gives evidence for the suppo- 

 sition that it represents a special anterior segment of the crani- 

 dium, which segment in the course of further development has either 

 fused with the doublure or been reduced to varying degrees. 



III. Development of the Mesonacidae. 



Of late years there have been described a large number of new forms 

 belonging to this interesting and ancient famil}^ of trilobites that plays 

 such an important part in the Lower Cambrian fauna. Walcott in parti- 

 cular has widened our knowledge by his magnificent and purposeful in- 

 vestigations, which to an almost inconceivable extent have brought a 

 number of fresh facts regarding the oldest fossil- bearing formations. His 

 excellent work "Olenellus and other Genera of the Mesonacidae", published 

 in 1910, gives a convenient survey of this family. He depicts and describes 

 30 species divided into lo genera, and discusses in addition their classi- 

 fication^ ontogenetic development, pedigree, and stratigraphie extension. 



In a later work ^ Walcott described 5 new species, so that with 

 the two new species of the present work we know nearly 40 species. 

 These fall into at least 11 different genera the majority of which have quite 

 distinct characters and often a definite stratigraphie occurrence, characterising: 

 special parts of the Lower Cambrian series. 



We should thus possess quite good conditions for obtaining a general 

 view of the phylogenetic development of the family. 



Walcott also gives in his work a general view as to how the latter 

 has in his opinion taken place -. 



According to Walcott, there is a line of development from the pri- 

 mitive Nevadia, which characterises the oldest part of the Lower Cambrian,. 

 through Mesonocis, Elliptoccphala and Paedeumias, to the typical Olenelhis. 

 The latter and markedly specialised genus marks the upper part of the 

 Lower Cambrian, but dies out without leaving any successors in the Middle 



1 New Lower Cambrian Sub-Fauna. Smiths. Misc. Coll. Vcl. 57 No. II, 1913. 



2 See especially page 249 and plate 44. 



