272 
Proceedings of the Royal Society of Edinburgh. [Sess. 
Like the floor of the perrnon cavity, that of the pleon lies wholly in 
one plane, and for this reason alone, apart from other peculiarities, it stands 
out distinct from the adjoining parts. It is narrower from side to side 
than the roof, the lateral walls in their ventral part sloping not only 
ventrally but medially. In consequence the bases of the pleopods are 
separated in the middle line only by a slight gap, which however widens 
as one proceeds caudalwards. Such medial approximation of the append- 
ages, still retained in the pleon though departed from in the perseon of 
Glyptonotus, is a primitive feature in Crustacea. 
Projecting ventrally from the middle of each of the three anterior 
sternites is a process, the medial sternal process, which on each lateral 
aspect carries a bearing surface for contact with the basal part of a pleopod. 
The least distinct of these three processes is the third, the most marked 
being the first ; the latter somewhat resembles the crista galli of the human 
ethmoid bone, and is arranged with its sharp edge in the sagittal plane. 
The pleopods are not only brought close to the middle line, but are 
also arranged in very close succession antero-posteriorly. The articular 
foramina in the sternites being relatively large, it follows that the sur- 
rounding calcareous skeleton is more reticular than solid in appearance. 
Although the tergites overlap, the sternites, where separate, are placed 
almost edge to edge, or more strictly the anterior border of the posterior 
segment slightly overlaps on the ventral aspect the posterior border of the 
anterior. The rigid bars that intervene between the successive articular 
foramina are exceedingly slim and narrow ; in the interests of strength 
they are deepened in the vertical plane, i.e. in the direction of muscular 
tension, and project prominently into the interior of the pleon cavity. 
Another and more marked internal vertical projection is formed above 
each calcareous side of the poststernal angle. 
As in the perseon, dorsal and ventral longitudinal bundles of muscle 
fibres move the somites on each other. The dorsal bundles lie just lateral 
to the median ridge on each side of the heart. Owing to lateral compres- 
sion of the pleonic floor the ventral bundles are much nearer the middle 
line than those in the perseon : when the pleopods are removed these 
muscular bundles can be seen crossing the medial portions of the articular 
foramina in the sternites ; traced caudalwards they deviate laterally in 
accordance with the more lateral situation of the posterior foramina. 
Curiously enough, these muscles persist posteriorly, although the correspond- 
ing somites have fused. They may have acquired new relations and uses, 
e.g. in connection with the anal apparatus. Of. here Flower’s (1891) account 
of muscles in the limbs of the horse. 
