16 BRITISH FOSSILS. 



two walls, as, even in the compressed fossil, the thickness of the 

 matrix filling the cavity sometimes amounts to one-sixteenth of an 

 inch. The third and fourth joints also had a considerable thickness. 



(c.) Of the third kind of paired appendages the most complete 

 specimens are those figured in Plate VI. fig. 1, and Plate VII. 

 figs. 1, 2. 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. 



The denticles or serrations form a single series, and diminish in 

 size from one end of the series to the other. The smallest is suc- 

 ceeded 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, which receives 

 the first joint of the long palpiform remainder of the appendage. 



The form of these joints is particularly described below (p. 75). 

 I will only remark here that there are six of them, and that the 

 penultimate, much larger than any of the others, is elongated, broad, 

 flattened, and widely emarginate at its distal extremity, where it 

 articulates with an oval palette-like plate, with serrated edges. A 

 brief comparison of the figures will satisfy the reader that this is the 

 part represented by Professor Agassiz in the lower right-hand corner 

 of Plate A (loc. cit.} 



From the form of the articulating edges of the joints of the pal- 

 piform 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. 



The Metastoma. The last kind of appendage (Plate VI. fig. 7) I 

 have to describe is perfectly symmetrical, and hence, even if there 

 were no other means of determining its real nature, its single or 

 azygos character might be divined. 



It is an oval plate with margins much thinner than the centre, 

 and presenting a median notch at one extremity. It is richly 

 sculptured, the facets having their convexities turned towards the 

 rounded entire end ; and it is worthy of notice that the facets are 



