Prof. H. H. Sivinnerton — Classification of Trilobites. 495 



making it the basis of his classification, and the names of some of his 

 sub-orders may be usefully adapted for describing the stages in this 

 process of caudalization. 



The earliest and most primitive stage is one in which the pygidium 

 consists of a telson or of a telson plus only two or three segments — 

 this may be called the micropygous stage. This passes into the 

 heteropygous stage, in which the pygidium includes a greater number 

 of segments, but is distinctly smaller than the cephalon. In the final 

 or isopygous stage the pygidium is approximately equal to or even 

 larger than the cephalon. 



Some or all of these stages may be represented in any genetic series 

 of Trilobites. The use of the number of free segments and the size of 

 the pygidium as the definitive characters of orders or sub-orders results 

 in the bringing together of forms which belong to quite different 

 lines of descent, e.g. Asaphidse and Phacopidse, 1 and separates others 

 which are closely allied, e.g. Arethusinidse and Proetidae. 1 



Suggested Subdivision of the Opisthoparia. 



The order Opisthoparia now contains such a great number of 

 families that some division into sub-orders is rendered necessary. One 

 distinct section, the Calymmenina, has been already recognized 

 (p. 494). The remaining families must now be studied with a view 

 to detecting, if possible, other equally natural groups within this- 

 order. 



Fig. 2. — Types of Lower Cambrian Trilobites. a. Nevadia (after Walcott, 

 1910, pi. xxiii) ; b. Conocoryphe (Conocephalites) sulzeri (after Bronn, 

 Klass, Ord., Bd. v, Arthropods, pi. xlv) ; c. Ellipsocephalus (after 

 Barrande). 



Already in the Lower Cambrian the genera JVevadia, 2 Cono- 

 coryphe, 3 and Ellipsocephalus exhibit distinct types. Though in one 

 or two respects each is specialized, in the general sum of their 

 characters they are each more primitive than any other allied genera. 



1 Vide p. 488. 



2 C. D. Walcott, Smiths. Misc. Coll., vol. liii, p. 256, 1910. 



3 Conocephalites sulzeri, Schlot. 



