﻿33 BRITISH FOSSIL CRUSTACEA. 



the palpus are much shorter, and thicker than the succeeding ones, which were subcylin- 

 drical and nearly equal in length for the next succeeding three articulations, ending with 

 two short joints, the terminal one being pointed and slightly curved. The charac- 

 teristic sculpture is visible upon a portion of the surface of the basal joint of these 

 appendages (see figs. 5 — 7). 



The Ectognaths, or Maxittipedes. — These are by far the most complete and well- 

 preserved of any of the appendages met with, and are represented by two basal joints, or 

 coxognathites (Plate I, fig. 3, and Plate IV) ; by a very large penultimate joint, or progna- 

 thite (Plate V) ; by an almost entire maxillipede, or ectognath (Plate VII, figs. 9, 10), all 

 of the natural size ; and lastly, by part of another example (Plate VII, fig. 11), three fourths 

 the natural size. 



These mouth-appendages, which occupy the most posterior position in the series (see 

 Plate II, figs. 1, 5, and Plate VIII, figs. 1, 6), and probably correspond to the first pair 

 of maxillipedes in the Decapoda, and the sixth pair of appendages in Limulus (see 

 Plate IX, figs. 1, 7), are evidently the most important and powerful manducating organs, 

 as well as being also the principal locomotory appendages. 



Prof. Huxley (op. cit., p. 16) thus describes an ectognath: — "It consists of an 

 exceedingly large and expanded, quadrate, basal joint, produced at one angle into a broad 

 curved process, which is obliquely truncated at its extremity. The truncated edge is 

 nearly straight, and is serrated, broad notches separating a number of strong, flattened, 

 pointed denticles, which are continuous with the substance of the joint, and not articulated 

 with it (see Plates I, fig. 3 m, and IV, m.) 



" The denticles or serrations form a single series, and diminish in size from one end of 

 the series to the other. The smallest is succeeded by the rounded corner in which the 

 truncated edge and the concave margin of the serrated process meet. 



" The surface of the joint and of its process is covered with the squamiform 

 ornamentation, and presents in the middle of the margin, opposite to the serrated process 

 a deep notch (Plates I, fig. 3 s, and IV, s), which receives the first joint of the long palpi- 

 form remainder of the appendage. This portion consists of six articulations, the penulti- 

 mate of which (Plate VII, fig. 10, g, and fig. 11, g) is much larger than any of the others, 

 is elongated, broad, flattened, and widely emarginate at its distal extremity, where it arti- 

 culates with an oval palette-like plate, with serrated edges. 



" From the form of the articulating edges of the joints of the palpiform part of this 

 appendage, I am inclined to think that, as in the chela of the lobster, the plane of motion 

 of each joint formed a considerable angle with that of its predecessor and successor, the 

 result of which would be a sort of feathering, or screw-propeller motion, of the ultimate 

 and penultimate joints during flexion of the limb." l 



In the recent state these appendages undoubtedly required for their effectual action the 



1 Such an arrangement, in fact, as is to be seen in the limbs of Swimming-crabs (see Plate IX, fig. 8). 



