*264 



in their number with the size of the .animal : the tail varying in its 

 size and form. 



The Dudley fossil, or that species of this fossil which is found at Dud- 

 ley, in Shropshire, is evidently the upper covering only of the animal, 

 and appears to have been of a crustaceous nature. It is of an oblong 

 ovate form, convex, and surrounded by an uninterrupted border. The 

 head is large and gibbous, and divided longitudinally into three parts : 

 the middle one rounded, gibbous, and rough, having at its posterior part 

 two round projecting knobs, and just before these two smaller. On each 

 side of this body is a triangular surface, from the centre of each of which 

 proceeds a valvular projection, which, from its form, appears to have 

 been capable of being occasionally opened or closed. 



I acknowledge that, in the specimens which I possess, I am unable to 

 discover the reticulated surface of the eye of this animal, of which 

 many have spoken. Instead of this, I only find the lunated valvular pro- 

 jection, by which, it seems, the eye of the animal might be occasionally 

 covered or exposed. A magnified representation of this part is given 

 Fig. 14,0. 



The back is formed of strong, convex, triarcuate segments, varying 

 in number with the size of the animal, and diminishing in size, as they 

 approach the caudal termination. These segments are more raised 

 in their middle than at their sides; and in the recent animal, the supe- 

 rior, by sliding over the inferior ones, allowed the animal to make very 

 considerable changes in its form, by extending or contracting itself) as is 

 shown Plate XVII. Fig. 11 and 14. The tail is obtuse, and without 

 any appendage. In no specimen has the under part of the animal been 

 seen, consequently nothing can be said respecting the structure of this 

 part, its legs, &c. 



The Derbyshire trilobite differs from that of Dudley in being nar- 

 rower, and particularly so at the upper part ; in not having the four tu- 

 bercles at the posterior part of the head, and in having the dorsal seg- 



