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 subjoimed Woodcut (fig. 11) conveys a correct idea of the general form 
Fie. 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 P?. 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 (vy), which are longest in front and have the appearance of 
being articulated at their base to the border of the coxal jot; the palpus is articulated 
to the mandible on its upper border by a short joint (4), succeeded by two nearly lnear 
joints of about equal length (¢ and m); the carpus (c), a very small articulation, is followed 
by the penultimate joint (), corresponding nearly in length with the third and fourth arti- 
cali; to this is articulated the slender, tapering terminal joint (7). The length of the palpi 
varies ; the longer of the two figured at Pl. XIV, fig. 3, measures 1 inch 7 lines from its 
articulation with the coxal joint to its distal extremity ; the shorter measures | inch 4 lines, 
whilst that figured in our Woodcut (fig. 11) is 1 inch 9 lines long ; the endognath (e) lying 
across the swimming-foot of the entire Pferygotus in Pl. X, fig. 1, measures 1 inch, 8 
lines. ‘'T'wo endognaths—one 7n sifii 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.. 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, pl. x, fig. 1, p. 197; and ‘Quart. Journ. Geol. Soc.,’ 1865, vol. xxi, pl. xii, 
p- 482. 
