36 BULLETIN OF THE NATURAL HISTORY SOCIETY. 



the pleurae become extended in length when the animal is fully grown. 

 Many of the trilobites of the Protolenus Fauna have short pleurae, 

 and none have been found to possess the prolonged pleura? which mark 

 many species of the Paradoxides and Olenus Zones (Primordeal). 



The development of a pygidium, or tail-piece of many joints, is 

 common in many trilobites of the Primordeal Fauna, and is still more 

 common in the Ordovician system above it. This part of the body- 

 covering is built of segments that were added gradually in the different 

 moults as the individual trilobite progressed toward maturity. A 

 pygidium of a few joints is therefore a primitive character, and the 

 entire subordination of this part to the headshield is a mark of simpli- 

 city of structure ; it is so throughout the Cambrian system*, but in 

 the next geological system (Ordovician) we sometimes find the pygidia 

 preserved in greater numbers and perfection than the head-shields (e. g., 

 Asaphus). As a contrast to this, it may be said that while there are 

 thirteen species of trilobites in the Protolenus Fauna, of which a 

 hundred or more of heads have been found, only one pygidium has 

 been recognized. The pygidia of many of these species must, then, be 

 small and insignificant. 



The primitive standing of the trilobites of this fauna is shown by 

 the following characteristics : 



1. The long, cylindrical glabella. 



2. The continuous eyelobe. 



3. The narrow movable cheek. 



4. The short pleura. 



5. The small pygidium. 



This fauna is remarkable for the variety and size of its ostracods, 

 and they differentiate the fauna into two sections — the lower in 

 Assize 1 marked by the presence of the genus Hipponicharion, the 

 upper in Assizes 2-4 marked by that of Beyrichona. These ostracods 

 are of comparatively large size, and, with other genera of this order, 

 there are a score of species. We seem to find this section of the crus- 

 taceans beginning to take the place of the trilobites in those oldest 

 Cambrian beds, and it may be the dominant order in older deposits. 



In this fauna we find ourselves among a very primitive assemblage 

 of Brachiopods, for among them are forms which it is difficult to assign 



'l'li.' two genera, IMicrodiscus and Lgnostus, may seem no1 to bear out this principle, 

 but it is i.. in- remembered thai these are abnormal genera, for in the former the joints of 

 the thorax may in- from two in four, while in the latter thej are strictly limited u> two; 



hence the pygidium enlarged bj additional - Ites, us a counterpoise to the enlarging 



beadshield. This provision enabled the animal to fold itself together and thus prol 

 soft under parts. 



