278 Proceedings of the Royal Society of Edinburgh. [Sess. 
with the external conformation of the animal, have commented upon the 
beautifully regular curved line by which the second true thoracic segment 
is evidently marked off from the parts in front. The curved line in 
question consists in its two lateral parts, i.e. where the carapace is smooth, 
of a groove ; in its medial part, where the carapace is sculptured and where 
complete intersegmental fusion has occurred, of a ridge — see fig. 13. The 
triangular expansions of the first intracephalosomic septum are attached 
along the course of each lateral groove. From about the region where 
each of the two lateral smooth areas on the head proper comes to a pointed 
Fig. 13. — Dorsal aspect of the “ cephalosome ” of Glyptonotus. Photograph. 
Natural size. 
The cephalosome proper is sunk into the second true thoracic somite, 
the separation between them consisting of a pair of lateral grooves and of a 
medial ridge. Just in front of the medial ridge and behind the sculptured 
part of the cephalon is a crescent-shaped area, which is the first thoracic 
somite. The deep transverse groove in front of this somite corresponds to 
the medial girder of the first intracephalosomic septum. 
end medially and posteriorly, the first intracephalosomic septum is con- 
tinued coronally across as the medial crescent-shaped plate, its position 
being now externally marked by a deep transverse depression behind the 
highly sculptured region of the head proper. 
For visceral, vascular, and nervous communication between head and 
thorax a wide medial foramen, the cephalo -thoracic foramen , is left inter- 
nally — see fig. 12. The inferior border or boundary of this foramen is not 
formed by the sternite, the opening being partially blocked in this region 
by a pair of structures, partly calcified, partly chitinous, which project 
upwards from a region just in front of the articular foramina for the 
maxillipeds and behind the corresponding foramina for articulation of the 
second, and especially of the first, maxillm. These structures, to which 
