Vol. 50.] SYSTEMATIC POSITION OF THE TRILOBITES. 4ol 



In this provisional classification we have assumed that Micro- 

 discus and Olenellus branched off, perhaps independently, from the 

 main stem, as forms specialized for creeping — by the development of 

 the pleura? along the whole length of the body. It is obvious, of 

 course, that there is an alternative scheme, namely, that which assumes 

 that Microdiscus and Olenellus stand more or less in the direct line, 

 and that Apus branched off from Olenellus (each having five head- 

 segments). Apus in this case would be a later specialization, charac- 

 terized by a failure to develop the pleurae (for example, the Eury- 

 pterids) along the trunk-segments, perhaps in adaptation to a more 

 free-swimming manner of life. In that case, its cylindrical vermi- 

 form body would be a return to ancestral conditions. 



The classification would then be as follows : — 



Browsing annelid, with first segment bent round, and lateral projections. 



With two segments fused to form the head, the lateral projections being 

 repeated all along the body as pleurae. 



With head of three segments, etc. The same. 



With head of four segments, e. g. Microdiscus and others. 



I 



With head of five segments, e. g. Olenellus and other trilobites with 

 five head-segments. 



Trilobites with six cephalic segments. Apus, in which the pleura? ar e 



secondarily limited to the head. 



i I segments, forming the head-shield' 



Eurypterids (also Limulus. which by backward prolongation 



with secondary de- becomes the carapace, 

 generation of the 

 pleurae). 



For my own part, I find the former classification the more 

 acceptable. The repetition of the head-shield as pleurae along the 

 trunk-segments, seems to be the specialization which characterizes 

 the trilobites. If Apus cannot show the primitive segmentation of 

 the head, no trilobite can show the vermiform body and the rich 

 segmentation of Apus. 



It seems to me, therefore, that the trilobites, studied in the light 

 of new discoveries, especially of those which we owe to American 

 investigators, yield the most interesting and important evidence as 

 to the origin of the Crustacea . Stripped of their pleurae and of the 

 expansion of the head-shield, we have, in the early trilobites (e. g. 

 Olenellus), long segmented animals tapering at the posterior end. The 

 first segment is bent round ventrally, so that the large labrum points 

 backwards. The appendages of the first segment appear to have 

 functioned as sensory organs and to have pointed downwards, being 

 inserted at the sides of the labrum. The following segments were 

 provided with membranous lobate appendages carrying, on their 



2g2 



