238 Description of a Species of Caligus. 
they diverge from their insertion and extend to the base of the — 
apron, (fig. 18.) Here they are united by aslender osseous process, 
which forms the upper limits of the sternum of the apron. They 
continue on, making at first one or two irregular curves, and form 
also the lateral boundaries of this sternum. By this remarkable 
arrangement, the articulation of the sternum of the large natatory 
legs is rendered sufficiently firm for their powerful action in the 
motions of the animal. 
The third pair of legs has been already stated to be expanded 
into the form of an apron, forming a broad lamellar appendage to 
the cephalo-thoracic portion of the body. This apron is composed 
of the same parts as the natatory last described, and there is an 
almost perfect coincidence in the number and nature of the ap- 
pendages. 
The sternal piece is very wide and lamellar. On its posterior 
margin there isa broad membranous expansion, identical in struc- 
ture and position with that appended to the preceding sternum. 
The portion of the apron corresponding to the small first joint of 
the natatory leg, is very narrow, and has very imperfectly defined 
limits ; we see an indication of its presence in the single pinnula, 
behind, adjacent to the sternum.* The second joint is expanded 
into a broad, irregular trapezoidal figure, with concave sides ex- 
cepting its posterior margin. Like the same joint in the natatory, 
it is furnished with a broad thin membrane posteriorly, and @ 
jointed appendage provided with pinnule. The first joint of this 
appendage is quite small, and bears a single pinnula as in the per- 
fect leg; the remaining portion is circular and is furnished with 
six pinnule. We find the analogue of the third joint of the 
natatory in a broad nearly circular plate, which is the lateral termi- 
nation of the apron; it is connected with the basal portion by an 
indistinct suture. The posterior margin of this joint, like the 
same joint above, bears a ciliated leaf-like expansion. The anal- 
ogy of the parts is still farther apparent in the strong articulated 
spine and pinnula attached to this joint, and in the two small termi- 
nal joints, furnished with pinnule ; the first with a single pinnula 
and a spine at its apex, and the second with four pinnule, and two 
short eee spines. 
* The correspondi: 
ng —_ of the apron, and the second of natatories, are in- 
dicated in the figure by the similar numbers in them. “= 
