PTERYGOTUS BILOBUS. 61 
is inserted a minute plate (w) which Prof. Hall calls a ‘ terminal palette ;’ this cannot, how- 
ever, be considered of greater importance than a spine or claw affixed to the distal 
extremity of the joint, like that seen on the tip of the modified antennule of the male 
Limulus (see Pl. IX, fig. 1 a), or on the extremity of the terminal oval palette of the 
swimming-foot of the recent Seyl/a serrata. 
The Metastoma, or Post-oral Plate.—This cordate plate is found with all the species. 
Its position is naturally behind the oral aperture, enclosing, with its anterior bilobed 
portion, the inner mandibular borders of the great coxal joints of the ectognaths (see 
Pl. VII, fig. 1 m; and Pl. X, fig. 2m). It also remains associated with the detached oral 
appendages figured in our Woodcut, p. 58, fig. 10 m. Its impression can be faintly seen 
through the head-shield of fig. 1 in Pl. VIII. Its length varies from 9 to 11 lines, and 
its breadth is usually about 5 to 6 lines. The anterior portion of the metastoma some- 
times displays upon its surface the characteristic semicircular plicee seen so readily in all 
the larger forms (Pl. III, fig. 1; Pl. XIII, fig. 1 e; Pl. XV, fig. 3). This plate was, no 
doubt, attached to the head by its posterior border, which, in some epee iets, is seen to 
be distinctly truncated at its lower end. 
The Thoracic Plate, or Operculum.—The normal position of this plate is upon the 
ventral surface of the body, covering the under surface of the first two thoracic somites ; 
it was attached along its anterior border to the posterior margin of the head; the rest 
of the plate was, no doubt, free, as in the recent Lamulus. 
I have somewhat fully described this plate in Part I, p. 39, and I shall have occasion 
to refer to it again when treating of Pt. perornatus, &c., so I will now only briefly 
describe its form in Pt. belobus. 
Fic. 12. Operculum of Pé. dilobus, var. 2. (Two forms seen.) 
a, a. Line of attachment to head. 1, 1. Lateral alee. e. Central appendage. 
It consists in this, as in all the other forms, of a median appendage and two lateral 
ale. ‘These alee, which are sculptured upon their outer and under side, are equal in length 
to the first two thoracic segments, and in breadth correspond exactly with the segments 
which they overlie. They are united in the median line by a narrow central lobe (rounded 
at its distal free end in Pl. X, fig. 1 e, but pointed in PI. X, fig. 2), hastate at its proximal 
end, which is directed forwards. 
I have drawn these two forms of plates on the accompanying Woodcut (fig. 12), in 
order that they may be more clearly seen. 
9 
