﻿PTERYGOTUS BILOBUS. 59 



They present no marked variation in the size of their basal joints, although the length of 

 the articuli of their palpi, no doubt, varies according to their anterior or posterior position 

 in the series. The subjoined Woodcut (fig. 11) conveys a correct idea of the general form 



Fig. 11. Endognath of Pterygotus bilobus. 



which they present. It is drawn from a detached specimen preserved in the British 

 Museum. 



Two other and more perfect detached examples (preserved in the Museum of Practical 

 Geology, Jermyn Street) are represented in PL XIV, fig. 3. 



In general form they closely agree with the far larger endognaths of Pt. anglicus (see 

 Part I, PI. VII, figs. 5, 6, 7). The coxal joint (co) has a broad tongue-shaped lamina (/), for 

 its attachment to the head, and is furnished along its inner free border with a row of about 

 nine or ten sharp, curved teeth (g), which are longest in front and have the appearance of 

 being articulated at their base to the border of the coxal joint ; the palpus is articulated 

 to the mandible on its upper border by a short joint (b), succeeded by two nearly linear 

 joints of about equal length (i and m) ; the carpus (c), a very small articulation, is followed 

 by the penultimate joint (p), corresponding nearly in length with the third and fourth arti- 

 culi ; to this is articulated the slender, tapering terminal joint (d). The length of the palpi 

 varies ; the longer of the two figured at PI. XIV, fig. 3, measures 1 inch 7 lines from its 

 articulation with the coxal joint to its distal extremity ; the shorter measures 1 inch 4 lines, 

 whilst that figured in our Woodcut (fig. 11) is 1 inch 9 lines long ; the endognath (<?) lying 

 across the swimming-foot of the entire Pterygotus in PL X, fig. 1, measures 1 inch, 8 

 lines. Two endognaths — one in situ on the left side of the head (e), and the other lying 

 across the three anterior thoracic segments on the right side — are seen with the 

 Pterygotus bilotus, PL X, fig. 2 ; whilst the basal joints of all three pairs of endognaths, 

 and portions of their palpi, are seen associated together in the specimen figured in our 

 Woodcut, fig. 10, on page 58. 



The Ectognaths in all the species of this order represent (so far as we are acquainted 

 with their appendages) the principal organs of locomotion and manducation. 1 They may 

 be considered to be homologous with the first pair of maxillipeds in the higher Crustacea, 

 and also to correspond with the last pair of appendages in Limulus (PL IX, fig. 1, 7). In 



1 Stylonurus is the only genus in which we find two pairs of elongated swimming appendages. See 

 'Geol. Mag.,' 1864, vol. i, pi. x, fig. I, p. 197 ; and 'Quart. Journ. Geol. Soc.,' 1865, vol. xxi, pi. xiii, 

 p. 482. 



