RECENT DISCOVERIES IN THE ST. JOHN GROUP. 33 



In studying the development of the Primordeal trilobites by means 

 of their young, we find certain features in the latter which are lost in 

 the adult. But among the trilobites of the more ancient Protolenus 

 Fauna these features are to be found subsisting in the full-grown 

 trilobite. 



One of these features is a long, cylindrical glabella (or middle lobe 

 of the head-shield). This in many of the larval trilobites is enlarged 

 in front, and so we find it in the adult of the genera Micmacca and 

 Ellipsocephalus of this fauna, showing how primitive these forms are. 

 Many of the trilobites have cylindrical glabellas (Avalonia and Proto- 

 lenus), which cylindrical shape is the second phase shown in the 

 glabella of the larval trilobites. Moreover none of the trilobites of 

 the Protolenus Fauna have contracted their glabellas to the short, 

 conical form prevalent in the trilobites of the Primordeal ^auna (Pty- 

 choparia, Conocephalites, Conocoryphe, Ctenocephalus, etc.) 



Another feature in the trilobites of the Protolenus Fauna is the 

 prevalence of a continuous eyelobe. The eyelobe is a protective 

 ridge extending along a portion of the seam between the middle 

 piece of the head-shield and the movable cheek. In many Cambrian 

 trilobites this lobe is quite short, but in others, while it is short in the 

 adult, it is longer in the larval stages ; this is notably the case in 

 Paradoxides, which in the earliest species, and in the young of all the 

 species studied, has continuous eyelobes — that is, eyelobes extending 

 to the posterior furrow of the head-shield. Now this character of a 

 continuous eyelobe marks all the trilobites of the Protolenus Fauna in 

 which this part of the headshield has been preserved. 



A narrow, movable cheek is apt to be associated with a continuous 

 eyelobe, and such movable cheek is the only kind so far found with 

 the trilobites of the Protolenus Fauna ; sometimes with, sometimes 

 without a genal spine. Such a cheek exists in the early larval exam- 

 ples of Pytehoparia and allied genera of the Primordeal Fauna, but 

 disappears in the adult, in which the area of the cheek is often quite 

 wide, owing to the withdrawal of the eye from the margin of the 

 headshield during growth. 



A fourth primitive character of the trilobites of the Protolenus 

 Fauna is the shortness of the pleur;e, or' lateral extensions of the 

 thoracic lings. This may be observed as a characteristic of many 

 trilobites in the larval state, but disappears as the species comes to 

 maturity. Ellipsocephalus and several Agrauli preserve this character 

 in the adult condition, but in the majority of Primordeal trilobites 



{See page 30). 



