BRITISH FOSSILS. 



Lastly, we have a very perfect head of the species, here given, 

 j. 8, which shows the palps in situ, and there is so strong a 



FIG. 8. / 

 P. acuminalus ; j 

 rather more than j 

 one-third natural 

 size. Lesmahago. | 

 Mus. Prac. Geol. \ 



Specimen show- 

 ing both first and 

 second pairs of 

 palpi, a, b, and the 

 swimming foot c. 



divergence between the front filament a, and the lower one 6, as- 

 to render it probable they are not mere branches of the same palp. 

 6 is far more imperfect than a. c, is the swimming foot (ectognath) 

 in situ. 



Swimming Feet, figs. 5 to 9. The great basal joints of these organs 

 are squarer in outline and^their serrate edges placed more vertically 

 (i.e. more parallel to the general direction of the plate) than in P. 

 bilobus, or P. anglicus, Plates I., VII., in order apparently to accom- 

 modate their shape to the elongate form of the head, The terminal 

 joints, too, of the limb are less expanded and more linear than in the 

 above-mentioned species. 



The size of the largest fragments would give about four inches- 

 and a half for the length by two inches and a half broad. The great 

 subquadrate basal joint occupies full two-thirds of the length ; it is 

 straight along the inner margin, rather square at the outer and 

 upper angle, rounded on the outer side, and has the notch for the 

 attachment of the remaining joints deep. On the lower third, behind 

 which the breadth lessens, the basal margin is convex. The entire 

 joint is thickened, and most so from the notch obliquely upwards ; 

 a ridge runs along the convexity, and another near the straight 

 inner edge. The terminal serrate portion is narrow-oblong, and set 

 very obliquely on the rhomboidal neck, from which it is obscurely 

 divided. Its upper (forward) margin is strongly arched, and the 



