282 Proceedings of the Poyal Society of Edinburgh. [Sess. 
length was found to be in the mid-dorsal region ; from this point lateral- 
wards its length was seen to decrease continuously, until the somite 
appeared to fade away where the second triangular lamina joins the first. 
In other words, it is wedge-shaped, and its wedge-like insertion between 
the cephalon and the second thoracic somite may be readily recognised 
in fig. 13. 
To external view the somite appears to end laterally at the groove 
between the two lateral smooth areas, cephalic and second thoracic 
respectively, on the dorsal aspect of the cephalosome. Inspection of this 
groove shows that it is merely the dorsal termination of a deep cleft — the 
(Lateral) cephalo-thoracic cleft — which cuts in medially from the lateral 
border of the cephalosome. Passing a bristle into this cleft and looking at 
the two triangular laminm on that side, one notes, thanks to the trans- 
parent thinness of the second lamina, that the point of the bristle moves 
freely in front of the latter, being capable of covering any point in its 
whole area right up to the medial free edge. In other words, this second 
lamina, forming the posterior wall of the cleft, is an inturning of the 
original free border of the lateral part of the second thoracic somite, or 
more properly is a portion of the coxa of the second thoracic appendage. 
Partly by the same method, partly by examination of the ventral part 
of the cephalosome and the interior of the mandibular articular foramen, 
one sees that the anterior wall of the cleft, co-extensive with the posterior 
wall, though of much less extent than the whole first triangular lamina, 
is formed by an inturning of the posterior free edge of the cephalosome 
proper. Is this a portion of the first thoracic somite, or can the latter be 
elsewhere identified ? 
Careful examination of the region in proximity to the free edge of the 
second triangular lamina shows that between this free edge and the surface 
of the first triangular lamina is another calcareous fold or reduplication 
exactly similar to the free edge of the second lamina and running all the 
way up and down parallel and closely contiguous to it — see fig. 16. The 
arrangement may be made more plain by means of a diagrammatic hori- 
zontal section through the cephalosome — see fig. 17. This reduplication, 
hidden at the bottom of the cephalo-thoracic cleft, and therefore far removed 
from the extreme lateral border of the animal, is evidently a remnant of 
the first thoracic tergite, which, therefore, cannot have participated in the 
general lateral expansion of the cephalosome. Comparison with Chiridotea 
entomon, Harger, in which the position of the first thoracic somite is 
better marked externally, confirms this view. 
Let us now turn to the ventral aspect. The posterior (or postero- 
