252 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 10, 



the neck-segment in all Trilobites ; the latter segment in our genus 

 is armed with a spine. 



rr^ 



Fig. 1. Trinucleus ornatus. 



2. Do. magnified fringe. 



3. Hypothetical development of the fringe. 



4. Fringe of T.fimbriatus, 



The body-rings are six, very flat, and capable only of bending or 

 doubling, not of rolling up as in Calymene — a character of considerable 

 importance, inasmuch as it has relation to the connecting membranes 

 and articular surfaces of the crust : species capable of rolling into a 

 ball requiring large articular surfaces on the segments to enable them 

 to move freely on each other ; and a fulcrum, as centre of motion, on 

 the side pieces of each segment, is generally placed about one-third of 

 the distance from the axis : — in Trinucleus the rudimentary fulcrum 

 is quite at the extremity of the segment, and apparently useless. 



The tail, generally smooth, has a deflected border, but not, as far 

 as observed, a striated internal fold, so common in higher forms of 

 Trilobites. 



The peculiar perforate border is the most interesting part of these 

 animals, and I propose to examine it critically. 



The puncta are almost always arranged in radiating rows, three, 

 four, or more holes in each row, and these being at equal distances 

 they form concentric lines. In T. granulatus^ two of these rows are 

 separated by a furrow from the rest ; in T. seticornis, three are distinct 

 from the remaining two or three, by the front rows being sunk in a 

 deep concentric furrow. Other modifications take place : in T. jimhria- 

 tus, the two front rows are turned downwards ; lastly, in T. ornatus — 

 for by that name we must call T. Caractaci — the dots occur most 

 frequently in quincunx order — i. e. the radiant rows appear zigzag, 

 and not direct (fig. 2) : this appearance is due to the great obliquity 

 of the rays (2, a a). I wish to call attention to this, because I con- 

 sider it enables us to understand the nature of the enigmatical puncta. 



If we suppose a head furnished with a produced membranous margin 

 (fig. 5) instead of a perforate one, we shall get at the explanation 



