418 



ME. H. M. BERNARD ON THE 



[Aug. 1894, 



may safely describe it as consisting of the fused lateral projections of 

 the cephalic segments. The first pair, I think, were the lateral pro- 

 jections which would naturally be formed by the bending round of 

 the first segment. This first pair of projections would give rise 

 to a second pair belonging to the second segment. I say ' would 

 give rise ' because, from the method of development of segmented 

 animals, the metameric repetition of special structures is a well- 

 known fact. We can thus suppose three pairs of ' pleurae,' 

 diminishing in size, developed on the 2nd, 3rd, and 4th segments 



Fig. 7. — Diagram showing the probable composition 

 of the head-shield. 



as metameric repetitions of the lateral projections of the first 

 segment. This stage seems indeed to be represented in the larval 

 Olenellus (fig. 3, p. 415), in which we have the head-shield composed 

 of the secondarily enlarged lateral projections of the first segment, 

 and three pairs of pleurae. These posterior pleurae of the posterior 

 developing head-segments slope directly backward, just as do the 

 pleurae of the posterior tail-segments, which are also rudimentary. 

 I consider this latter point of great morphological importance, as it 

 seems to show that the head-shield was a structure sui generis. 



This head-shield, composed of the pleurae of four segments, in the 

 same way gave rise in the trilobites to large pleurae on the 

 subsequently developed trunk-segments, these pleurae generally 

 diminishing in size from front to back. If the first pair of these 

 pleurae fuse with the head-shield, as above described, we should get 

 a head-shield composed of (1) the lateral projections of the first 

 segment, (2) the pleurae of the second, (3) the smaller pleurae of the 

 third segment, (4) the still smaller pleurae of the fourth segment, 

 (5) the pair of the large pleurae of the most recently incorporated 

 trunk-segment forming the fifth cephalic segment. This origin is 

 further illustrated by the diagram (fig. 7). That diagram finds ample 

 justification in the series of figures 3, 4, 5, and 6, in which we trace 

 the rise of the fifth cephalic segment, with the gradual develop- 

 ment and incorporation into the head-shield of its pleurae, which 

 are typical trunk-pleurae. 



