DEVELOPMENT 



239 



indications of five cephalic segments ; the eyes, when present, 

 are found at or near the front margin, and the free cheeks, if 

 visible at all on the dorsal surface, are narrow. For this early 

 larval form Beecher has proposed the name " protaspis " ; he 

 regards it as the representative of the Nauplius of other 

 Crustacea, but that view is not accepted by Professor J. 8. 

 Kingsley. 1 



The general changes which occur in the course of develop- 

 ment are : modifications in the shape and relative size of the 

 glabella, and of the number and depth of the glabella-furrows ; 

 the growth of the free cheeks and the consequent inward move- 

 ment of the facial sutures arid eyes; the introduction of and 

 gradual increase in number of the thoracic segments, and the 

 relative decrease in size of the head. 



Sao hirsuta is a species found in the Cambrian, the develop- 

 ment of which was fully described by Barrande. Its earliest 

 protaspis stage (Fig. 143, A) is circular in outline ; the glabella 

 expands in front and c B A 



reaches the anterior 

 margin; the pygidial 



region is not dis- /<&9iKK)U o 



tinctly separated 

 from the cephalic 

 region ; segmenta- 

 tion is indicated in 

 the former, and the 

 neck-ring is present 

 in the latter ; the 

 eye-line is seen on 

 each side of the 

 glabella near the 

 anterior margin. In a later stage (Fig. 143, C) the segmentation 

 of the glabella becomes more distinct, indicating the existence of 

 five cephalic segments, and the facial suture appears near the 

 margin limiting a very narrow free cheek. Subsequently (Fig. 

 143, D-F) the thoracic segments develop, and increase in 

 number until the adult stage (G) is reached; also the eyes appear 

 at the margin of the cephalic shield,' and gradually move inwards, 

 and the glabella becomes narrower and rounded in front, and ceases 



1 American Geologist, xx., 1897, p. 34. 



FIG. 143. Development of Sao hirsuta, Barr. Cambrian. 

 A, Protaspis ; B-F, later stages ; G, adult. The small 

 outlines below each figure show the actual size of each 

 .specimen. (After Barrande.) 



