270 Proceedings of the Boyal Society of Edinburgh. [Sess. 
on each side of the medial ridge. The sixth segment, as wide as the telson, 
is by much the shortest. 
The simplest way to appreciate the skeletal structure of the pleon in 
its ventral aspect is to remove the internal contents and to disarticulate 
the pleopods. * 
The under surface of the pleon, or more properly the floor of the pleon 
cavity — see flg. 10 — presents features complementary to those just described 
as visible from the dorsal aspect. Segments 1, 2, 3, and 4, between which 
A B 
Fig. 10. — Two photographs of the ventral aspect of the pleon of Glyptonotus after 
removal of the appendages. Natural size. 
In A the circumanal cuticle has been left in situ. Owing to drying of 
the preparation the anal valves, which naturally lie in apposition in the 
middle line, are open, and the anus appears larger than usual. 
In B the circumanal cuticle, including the anal valves, has been removed 
to display the exact extent of the calcareous skeleton. 
lie the sole movable articulations, are the only ones provided with com- 
plete sternites. The fifth sternite has no middle part, its lateral calcified 
portions reaching forward in fusion with the corresponding parts of 
sternite 4. Thus the fifth sternite gains attachment to a medial portion 
only by proxy, much as the eighth costal cartilage in man gains attachment 
to the sternum. No calcified part in this region * can be identified as a 
remnant of the sixth primitive sternite. The ventral surface of the telson 
proper, as in all Crustacea, is devoid of calcification. 
The under skeleton of the pleon cavity curiously reminds one of the 
skeleton of the human chest as seen from the front, for the anterior 
* As we shall see later, the calcareous sternite of the sixth segment has not wholly 
disappeared, though it takes no part in formation of the floor of the pleon cavity. 
