79 



arch • the outer posterior angle is prolonged in the form of a small 

 spinJ Near the glabella there is a large pear-shaped mark mdi- 

 cating the position of the eye, which appears to have occupied 

 nearly a third of the cheek. Axis about one-third wider than the 

 pleura, and tapering gently towards the pygidium; there are 

 seven well-defined thoracic segments, and it is probable that the 

 number may have been eight: they are faintly-arched, and the 

 ends adjoining the axal furrows are well rounded. The front 

 sec^ments measure nearly half an inch in width, but they are a 

 Utile narrower towards the pygidium. The segments of the pleura 

 correspond with those on the axis, and maintain the same curve, 

 they are also well-rounded at the end adjoining the axal furrow, 

 but are obtusely-pointed at the outer end. 



Locality— Randel Crag. From my own cabinet. 



The deposition of the slate at Randel Crag must have been 

 under conditions unusually favourable for the growth and develop- 

 of animal life, and also for the fossilization of their remams, as 

 many of the best-developed and most perfectly-preserved tnlobites 

 and graptolites have been found there. 



The fossil remams of trilobites differ very much from those of 

 the graptolites occurring in the same beds; the latter, when 

 obtained in relief, always consist of iron pyrites, which has replaced 

 the original chitinous shell of the organism, and the shell appears 

 to have been of equal hardness on every side ; but the remains of 

 trilobites almost invariably consist of slate of the same type as 

 that in which they are enclosed, and it is the dorsal surface alone 

 that has been fossilized. It is evident, therefore, that the shell on 

 the ventral surface of the body has been of a very fragile character, 

 and that in almost every case it has perished, together with the 

 legs, swimming appendages, and internal organs of the creature, 

 the dorsal shell alone being sufficiently hard to retain its form 

 until the fine mud of which the Skiddaw Slate is formed became 

 •soUdified above and beneath it; hence, when the slate breaks up 

 along the bedding planes, we find a cast only of the whole or a 

 part of the internal or external surface of the shell. The shell 



