Vol. 51.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxxi 



Walcott, in the Geological Magazine, 1 and H. M. Bernard, in the 

 Quarterly Journal of this Society, 2 have given valuable additional 

 contributions on the appendages of trilobites and on their systematic 

 position, which are most helpful and acceptable at this time. 



The first point insisted upon by all systematic zoologists — long- 

 before the finding of appendages had thrown so much new light 

 upon our investigations — was that the great variability in the 

 number of the segments in trilobites was a feature which distinctly 

 connected them with the phyllopoda. 



Bernard considers of greater importance still the gradual dimi- 

 nution of the size of the segments posteriorly, which remarkable 

 feature the trilobites share with Apus. 



I would also call attention to the fact that those earlier trilo- 

 bites which best exhibit this large number of segments, such as 

 Olenellus, Paradoccides, etc., are likewise remarkable for the sim- 

 plicity and exact similarity of their segments, being a serial 

 repetition of one another, and even the coalesced segments forming 

 the head-shield share the same resemblance with the free posterior 

 thoracic and abdominal ones. 3 



This was long ago designated by Owen as ' the vegetative repe- 

 tition of parts,' and is most completely exemplified in the annelida 

 and the myriopoda. 



Bernard has given expression to the idea most aptly when he 

 writes (op. cit. p. 412) : — ' The adult is but the grown, not meta- 

 morphosed, larva — grown by the continual development of segments 

 from before backwards, until at a certain stage this process becomes 

 fixed, and we have the adult Apus with a number of fixed rudimentary 

 segments. This fixation of a number of undeveloped segments is 

 visible also in many trilobites.' 



In the earlier forms (as Olenellus) these rudimentary posterior 

 segments still remain free ; but as a rule they are coalesced to 

 form the plate-like pygidium so characteristic of the trilobites. 



Turning to the appendages, the simple multisegmented flagellate 

 antennae are extremely characteristic of the Crustacea, being met 

 with in lowly copepods and highly developed decapods. 



The biramose paired limbs are quite a primitive type, like the 



1 1894, pp. 246-251. - August 1S94, vol. 1. pp. 411-434. 



8 Triarthrus is essentially a ' Primordial ' type that has retained its general 

 simplicity of structure, although it has continued on until Upper Ordovician 

 time. 



