84 JOHAN KIÆR. M.-N. Kl. 



Cambrian. Another line proceeds from Nevadia to genera like Callavia^ 

 Holmia, and Wanneria, from which again the Middle Cambrian Paradoxidae 



has developed. 



The best established of these lines is the former. Extremely interesting 

 is Walcott's remarkable discovery of Paedeiunias transitans Walc. which 

 constitutes a most beautiful transition from Mesonacis to Olenellus. It may 

 be assumed that this is a real series of phylogenetical stages. 



On the other hand I cannot perceive wh}^ Elliptocephala should con- 

 stitute a middle stage between Mesonacis and Paedenmias. To judge from 

 the nepionic stages of its development it is most probable that it descen- 

 ded from ancestors resembling Mesonacis; but several points of its struc- 

 ture appear to indicate that it should for preference be regarded as a 

 lateral line diflfering from the Paedeuniias-Olenelhis line. This is indi- 

 cated by the structure and form of the pleurae, and the greater reduction 

 of the fully developed thoracic segments. (13). 



As far as Xcvadia is concerned, Walcott's discovery of this form is 

 of the greatest possible interest, as it gives us an idea of the development 

 in the oldest Lower Cambrian of Mesonacidae. It is clearly a very primitive 

 form of the Olenellus line, without however constituting the real ancestor of 

 this line. As indications of this we have the remarkable form of the 

 pleurae shown from the following considerations. The ontogenetical deve- 

 lopment both in Paedetimias and Olenellus indicats an evolution from 

 transsected pleurae to long falcate ones. This agrees with the condition 

 in the more primitive Mesonacis in which the pleurae extend into pleural 

 spines that are cut off obliquely. 



In the still more primitive Nevadia we should thus expect to find a 

 si^iilar form of the pleurae as in Mesonacis. The former are however elon- 

 gated into long, curved falcate spines and appear therefore not to represent 

 a primitive stage of development. 



Nevadia must therefore be assumed to constitute a lateral line to the 

 original main stem. 



The change in form of the pleurae here discussed must be assumed 

 to be due to the circumstance that the creatures in the course of their 

 development have altered their biology. They must have passed from a 

 nectonic to a more bentonic life on the mud of the litoral zone. The most 

 adapted to this state is the last stage of this line of development, viz. the 

 typical Olenellus forms, with their long spineshapfed telson, which curiously 

 enough according to Walcott's discovery does not represent the pygidium 

 but the greatl}' modified 15th segment. We have thus a distinct parallel 



