310 
MR. WILSON ON THE STRUCTURE AND DEVELOPMENT OF 
the second and third pair of legs, also forming a gentle curve, whose convexity i$ 
backwards. The fourth pair of ridges proceed obliquely outwards and backwards 
on each side, and terminate at the groove between the third and fourth pair of legs. 
There is no line of separation between the thorax and abdomen, and the median 
ridges gradually diminish in their prominence on the last thoracic segment, until at 
the boundary of the latter they are lost. The spaces of the breast included between 
the transverse ridges in front and behind, and the median ridge and base of the 
legs laterally, are irregularly quadrangular in form, and increase in breadth from 
before backwards in consequence of the greater separation between the legs. 
The lateral and dorsal portion of the thoracic segments is marked on its surface 
by a number of transverse lines situated at minute but regular distances. The spaces 
between these lines represent so many narrow plates which are unbroken from the 
base of the lateral portion of each thoracic segment across to a similar point on the 
opposite side, and overlap each other slightly from before backwards. On the con- 
vexities of the thoracic segments on each side, near their bases, the plates converge 
and become more imbricated than in their upper portion. By this arrangement a 
small triangular interval is left in the concavities of the segments which is occupied 
by short supplementary plates. 
The thorax admits of flexion to a limited extent, both in the antero-posterior and 
in the lateral direction, and of a slight degree of retraction in the longitudinal axis. 
The legs are four in number on each side ; in general form they are conical, broad 
at their bases, where they are attached to the side of the under part of the thorax, 
and truncated at their free extremities. They are composed of three segments ; the 
proximal segment covers by its articulation a considerable expanse of surface, extend- 
ing for some distance inwards upon the breast, and outwards upon the side of the 
thorax. In longitudinal extent it occupies very nearly the entire breadth of the 
thoracic plate. This segment resembles in its form the base of a cone obliquely 
truncated from before backwards ; its anterior depth being considerable and the 
posterior very narrow. On the outer side of this segment, and along its base and 
inferior border, are situated several spinous tubercles, and at the extremity of the 
anterior margin is one of larger size than the rest ; it is curved backwards. The 
proximal segment is articulated with the thorax by means of a loose capsular mem- 
brane, which permits of a trifling degree of retraction of the segment within the 
cavity of the thorax, and also allows of a slight degree of motion in all directions ; 
the principal movement of the segment being forwards and backwards. The middle 
segment is a cylindrical piece articulated with the preceding by means of an exceed- 
ingly loose capsular membrane. Its movements are flexion and extension in the 
antero-posterior direction, and retraction to a considerable degree. The antero- 
posterior movements are very extensive ; the former almost buries the middle in the 
superior segment, approximates the third piece and recurved tubercle of the anterior 
border of the proximal segment, and performs a sharp, angular flexion posteriorly ; 
