PROSOMATIC SEGMENTS OF LIMULUS 



251 



completely hidden by the prosomatic carapace, or became so soft as 

 not to be preserved in the fossilized condition. The appearance of 

 the prosomatic carapace is, to my 

 mind, suggestive of the presence of 

 such appendages, for it is marked out 

 radially, as is seen in the figure, in a 

 manner resembling somewhat the mark- 

 ings on the prosomatic carapace of 

 Mygale or Phrynus ; the latter mark- 

 ings, as already mentioned, are due to 

 the aponeuroses between the tergo-coxal 

 muscles of the prosomatic appendages 

 which lie underneath and are attached 

 to the carapace. 



A very similar radial marking is 

 shown by Woodward in his picture of 

 Hcmiaspis limuloidcs, reproduced in Fig. 

 109, found in the Lower Ludlow beds at 

 Leintwardine. This species has yielded 

 the most perfect specimens of the genus 

 Hemiaspis, which is recognized as differ- 

 ing from Bunodes by the possession of a 

 telson. 



It is striking to find that similar 

 indications of segments have been found 



Fig. 109. — Hemiaspis limuloidcs. 

 (From Woodward.) 



<]L, glabelluui. 



on the dorsal surface of the head-region 



in many of the must ancient extinct fishes, as will be fully discussed 



later on. 



The Evidence of Ccelomic Cavities. 



In the head-region of the vertebrate, morphologists depend largely 

 upon the embryonic divisions of the mesoderm for the estimation of 

 the number of segments, and, therefore, upon the number of ccelomic 

 cavities in this region, the walls of which give origin to the striated 

 muscles of the head, so that the question of the number of segments 

 depends very largely upon the origin of the muscles from the walls of 

 these head-cavities. It is therefore interesting to examine whether a 

 similar criterion of segmentation holds good in such a segmented 



