THE GENUS PTERYGOTUS 179 
presenting the least approximation to the structure here described, 
and the true carapace of Péerygotus has already been described. 
I shall endeavour to prove by and by that this singular plate is 
in fact the homologue of the epistoma and labrum of other Crustacea. 
3. Lhe Body Segments ——Numerous oblong plates and impressions 
of such, ornamented more or less extensively with the peculiar 
sculpture, have been discovered (Plates TV. and V.) [Plates 15, 16]. 
Of these, some are much elongated transversely, and, if we may be 
guided by the analogy of P. dzlobus and perornatus, belonged to the 
anterior part of the body. Others are longer than they are broad, 
and it may be concluded, on similar grounds, that they represent 
posterior segments. In the specimen figured in Plate IV. [Plate 15] 
fig. 6, two such segments remain in their natural relation to one 
another, but it is, unfortunately, impossible to ascertain whether they 
simply overlapped one another and were merely connected by 
membrane, or whether their union was effected by a more definite 
articulation. In Plate IV [Plate 15] fig. 1, a segment is seen 
to send a strong process forward from its anterior and external 
angle, and a similar structure appears to have obtained in the 
specimen figured by Professor Agassiz, in the right-hand upper 
corner of his Plate A (doc. czt.). This process may have served to 
articulate the segment with its predecessor, or it may have given 
attachment to muscles. 
The specimen figured in Plate V. [Plate 16] fig. 2, would at 
first appear to indicate that the segment had a sub-circular or 
elliptical section, but it is so much crushed and distorted, that I am 
not inclined to lay any great stress upon the conclusions which may 
be drawn from it, the more especially as all the other specimens 
which give a view of the thickness of the body rather lead to the 
belief that it was considerably depressed, at any rate, posteriorly. 
The segment, probably penultimate, figured in Plate V. [Plate 16] 
fig. 3, for example, exhibits no signs of having undergone any very 
considerable compression, and yet its thickness does not equal 
more than one-fourth of its breadth. It is quite possible, and indeed 
probable, however, that the proportions of the antero-posterior and 
transverse diameters may have been different in the anterior and 
posterior segments. . 
It is probable that the segments were sculptured on the ventral, 
as well as on the dorsal surface, in all cases, as that last referred to 
certainly is so. No segment has as yet exhibited any trace of an 
appendage or of an articular surface for one. 
A specimen in the possession of Mr. Lightbody exhibits at least 
N 2 
