﻿PTERYGOTUS BILOBUS. 



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is inserted a minute plate (u) 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 PI. IX, fig. 1 a), or on the extremity of the terminal oval palette of the 

 swimming-foot of the recent Scylla 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 

 PI. VIII, fig. 1 m ; and PI. X, fig. 2 m). 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 PI. 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 plicae seen so readily in all 

 the larger forms (PL III, fig. 1 ; PI. XIII, fig. 1 e; PI. XV, fig. 3). This plate was, no 

 doubt, attached to the head by its posterior border, which, in some specimens, 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 Limulus. 



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. bilobus. 



Fig. 12. Operculum of Pt. bilobus, var. a. (Two forms seen.) 

 a, a. Line of attachment to head. I, I. Lateral alse. c. Central appendage. 



It consists in this, as in all the other forms, of a median appendage and two lateral 

 al*. These alae, 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 PI. X, fig. 1 c, 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. 



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