﻿60 BRITISH FOSSIL CRUSTACEA. 



the larval Decapod we find locomotion effected by the enlargement of two pairs of the 

 maxillipeds, the true thoracic limbs being undeveloped, the abdomen being also des- 

 titute of appendages, thus affording another illustration among recent Crustacea of this 

 palaeozoic type of structure (see PL IX, fig. 7). 



The ectognaths in Pterygoids bilobus, var. a, differ but little in general form from those 

 of the great Pi. anglicus described in Part I of this Monograph ; they can be seen in situ 

 in figs. 1 and 2, PL X, and in the detailed drawing of the detached appendages of fig. 3 

 given in the Woodcut at page 58 (fig. 10). 



The following is an enumeration of the several joints of this appendage : 



1. Basal joint [coxa). — The great lower lobe is wide and of a spherico-triangular 

 shape, the inner margins, as well as the outer and base, being all convex. The neck is 

 suddenly contracted and short, and the serrated terminal lobe transverse or oblong, greatly 

 oblique, and overlapping (in many specimens) the opposite edge. 



Its toothed margin is curved and set with about thirteen small teeth. 

 The succeeding joints (which together make up the swimming-foot) are articulated 

 to the posterior outer angle of the great lower lobe of the coxal joint. 



2. Basos, short and broad linear. 



3. Ischium, narrow, longest on the inner margin. 



4. Meros, longest on the outer margin ; lower border indented, forming an articulation 

 for the — 



5. Carpus, somewhat triangular in form, having its outer and longest border convex, 

 and its upper edge produced so as to form a process by which it is more strongly articu- 

 lated with the fourth joint. 



6. Propodos. — With the exception of the great basal joint, this is the largest of any, 

 being equal in length to the four preceding articuli. It is oblong, but contracted slightly 

 at the upper end, so as to give a curved outline to its outer border ; the upper or proximal 

 end is doubly curved where it is articulated with the fifth joint ; the inner lateral margin 

 is nearly straight ; the distal end is pretty deeply indented, so as to form a rounded notch 

 for the reception of the terminal joint. 



Between this joint and the terminal one there is what Prof. Hall has called 1 an inter- 

 calated triangular plate (see Woodcut, p. 58, fig. 10, 5, i), united to the border of the 

 penultimate joint, and overlapping the oval terminal palette (see ante, Part I, p. 35). 



Dactylos. — A regularly ovate palette, as broad as the penultimate joint, and nearly as 

 long ; it fits into the bilobed extremity of the sixth joint, and is held fast by its articu- 

 lation with it, and by the overlapping of the intercalated triangular plate (i) already 

 referred to. 



This ovate palette is seen to be indented at its distal end, and into the indentation 



1 See 'Nat. Hist, of New York, Palaeontology,' vol. iii, 1859, pp. 397 and 400, on Eurypterus, pi. 80 

 and nine following plates. 



