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 gl. 
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 
Hemiaspis limuloides, 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. 
éqsa id Frg.109.—Hemiaspis limuloides. 
It is striking to find that similar (From Woopwazp.) 
indications of segments have been found 
on the dorsal surface of the head-region 
in many of the most ancient extinct fishes, as will be fully discussed 
later on. 
gl., glabellum. 
THE EVIDENCE oF C@LOMIC 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 
