Prof. H. H. Swinnerton — Classification of Trilobites. 493 



a posterior facial suture such, as has been seen apparently in some 

 Mesonacidse is present, but no well-established and clearly denned 

 line such as characterizes tbe majority of Trilobites occurs. 



For the reception of Trilobites and Trilobite-like organisms in which 

 the absence of facial sutures is primary the Order Protoparia may be 

 instituted. From this protoparian stock there arose independently, 

 as will be seen, several widely different types with free cheeks. The 

 position in which the suture appeared in the earliest representatives 

 of each of these types depended upon the position of the eye, whether 

 it was near the glabella, or near the margin, or, conceivably in some 

 cases, on the ventral side. 



The Interrelationships of the Pro- and Opisthoparia. 



The predominance of Opisthoparia in the Cambrian and of the 

 Proparia in later rocks suggests that the latter descended from the 

 former. This is apparently supported by the existence of the families 

 Calymmenidse and Homalonotidse, in which the suture cuts the margin 

 at the genal angle and thus seems to furnish a transition from one to 

 the other. 



FlG. 1. — Diagrams to illustrate differences in mode of origin of free cheeks of 

 Opisthoparia and Proparia. a. Head -shield of Conocoryphe (from Beecher) ; 

 b. metaprotaspis of Ptychoparia kingi (from Beecher, Amer. Geol., 1895, 

 p. 171) ; c. head-shield of Anacheirurus (Cheirtirus) frederici (recon- 

 structed after Salter, Mon. Pal. Soc, pi. v, fig. 18) ; d. head-shield of 

 young Dalmanites (from Beecher) ; e. Burlingia (after Walcott). 



The discovery of Burlingia 1 (Fig. \e) as early as the Middle 

 Cambrian throws doubt on this view of the interrelationships of 

 the two orders. This form is in many respects as primitive as the 

 Mesonacidse and yet is as typically proparian as the most specialized 

 of the Cheiruridse. On the other hand, the families Calymmenidse and 

 Homalonotidse do not appear before the Tremadocian. 



If Proparia descended from Opisthoparia evidence should be forth- 

 coming from their development. But such evidence is altogether 

 lacking. The development of the free cheek follows quite different 



1 C. D. Walcott, Smiths. Misc. Coll., vol. liii, p. 14, 1908. 



