PTERYGOTUS BILOBUS. 57 
At first sight it would appear that the body of Pt. dilodus, var. a, was destitute of the 
characteristic markings discernible upon the larger examples of this genus ; but such is 
really not the case, for although not always readily seen, they can, with the aid of a good 
pocket-lens, usually be detected upon some part of the body- a 
. : : . ° Sy SS 
segments. It consists in this, as in the larger, species of numerous IRE 
i ; KPO 
small semicircular plicee (the curve opening forwards, see Woodcut), SOP 
. 3 n vy Saw 
which probably covered the anterior half of each segment, leaving eco 
the posterior destitute of ornament (see Pl. XIII, fig. 1). re: os Onmatene 
The head in this as in the other species is destitute of EO 
: of Pt. bilobus. 
scul’sture on its surface. a. Anterior border. 
We must now proceed to the examination of the under side of the head and the 
appendages. 
Numerous specimens belonging to this species can now be seen and studied, both 
in the Museum of Practical Geology, Jermyn Street, and in the British Museum, in 
which many of the appendages still remain zz sé¢é ; whilst in other examples they are 
found displaced, but preserved upon the same slab with the entire body ; from which it is 
evident they have become detached by the quiet dissolution of the softer muscular tissues 
of the animal. 
In the accompanying Woodcut (p. 58) [have delineated, of the natural size, the oral append- 
ages belonging to the entire specimen of Plerygotus bilobus, var. a, drawn upon PI. X, fig. 3. 
These (by some slight movement in the fine, soft, clayey sediment in which the animal was 
enveloped) must have become separated from the rest of the body before decomposition 
had proceeded very far; for, although fractured and imperfect in detail (by reason of the 
present indurated character of the matrix), they still clearly prove that the entire buccal 
apparatus—held together by its integument—has thus been preserved to us. 
The separate organs are represented as they lie upon the surface of the same slab of 
shale in which the entire body (Pl. X, fig. 3) is preserved. Their correct order and 
position may be readily understood by turning to Part I, Pl. VIII, fig. 1, where I have 
given a restored figure of the under side of the great P?. anglicus, with its oral appendages 
in place. The antenne (Woodcut, fig. 10, 1,1; Pl. X, figs. 1, 2, a, a) are about 27 inches 
in length by } inch in breadth at the base of the chele, the moveable ramus being about 8 
lines long. Both the fixed and the moveable rami of the chele are slender ; the former is 
slightly shorter than the latter; their points are sharply incurved; the inner borders are 
armed with minute teeth, which, like that of the larger species, vary in size, being at 
intervals marked by one much larger than the rest. 
There appear to be five joints in the antenne of this species, but it is seldom that their 
true line of articulation can be readily distinguished. Five are clearly to be seen in one 
of the antennze figured in the accompanying woodcut, fig. 10. 
