174 F. R. Cou-per Reed — On the Rhacoindw.. 



replaced by a new one if the morpbological or pliylogenetic claims 

 of the group which it marks are sufficient to warrant a distinctive 

 title. 



The divisional names, therefore, which remain after this pre- 

 liminary weeding-out are the following : — 



' Aeaste.' Monorahos. 



Asteropyge. Odontocephalus. 



Chasmops. Fhaeops. 



Coromira. Porthckia. 



Corycephalus. Frobolium. 



Gryphina. Pterygometopus. 



Dalmanites. Somatrikelon. 



Homalops. Synphoria. 



Malladaia. T rimer ocephalus. 



Whether all the above divisions have sufficient morphological or 

 phylogenetic value to be retained in a natural system of classification 

 is the question now to be discussed. 



Pi-inciples of Classification. 



Dismissing from our minds all ideas and prejudices derived from 

 customary usage and past authorities, it is worth while considering 

 in what manner we should group the members and species of the 

 Phacopidse supposing we were to start completely de novo. The 

 principles for a natural arrangement are acknowledged to be found 

 in the modifications of the head-shield, as we have learned especially 

 from Professor Beecher's valuable researches. In the case of the 

 members of a compact familj^ such as the Phacopidte the changes in 

 the glabella ofii'er the best guide, and these changes are shown by 

 the fusion or reduction or even disappearance of certain lobes. The 

 modifications in the thorax, which always retains the fixed number 

 of eleven segments, are of secondary morphological value and of 

 minor developmental importance ; and while there is an immense 

 amount of variation in the characters of the pygidium in shape, 

 number of segments, and ornamentation, yet the evidence of the 

 trilobites as a whole and of individual families (e.g. the Cheiruridae) 

 indicates that structural changes in this part of the body occur in 

 less regular succession and pursue often an independent or eccentric 

 development, so that for the purposes of a classification which is 

 intended to be based on phylogeny they are of less significance than 

 those in the head-shield. 



Stai'ting, then, from the well-established view of the primitive 

 pentamerous composition of the head-shield of trilobites, we may 

 expect to find this pentameric segmentation best exhibited in the 

 earliest and least modified members of the family. The phylogeny 

 may be anticipated to follow the ontogeny which has been traced in 

 more than one species of Phacops. 

 \> The group termed Dalmanites by Barrande, using the name in its 

 widest and most comprehensive sense, most clearly preserves the 

 fivefold division of the head-shield. In the early post-embryonio 

 stages of its species, as well as at maturity, the five segments are 

 well displayed. The retention of this primitive character throughout 



