72 



but differ from it in having a comparatively-flat glabella, whereas 

 Mr. Salter's specimen is remarkable for its convexity. 



The features of trilobites on which authors chiefly base their 

 classification, are the perfect or imperfect development of the eye, 

 the form and position of the facial suture, the number of body 

 segments, and the relative size of the pygidium. In our Skiddaw 

 Slate specimens, the eyes and facial sutures are either indistinct or 

 altogether absent ; therefore, any classification, if attempted, must 

 be based on other features; but, as the materials at hand are 

 meagre and imperfect, it would be premature to name or to classify 

 them until we obtain a larger assemblage of their fossil remains, 

 I shall, therefore, merely group them together in the following 

 order, namely : — 



A. Trilobites having the thoracic portion of the axal furrows 

 more or less curved, caudal shield doubtful or altogether absent. 



B. Forms having the thoracic portion of the axal furrows more 

 or less curved, and a well-developed caudal shield. 



C. Those having the axal furrows straight, except the outward 

 curve near the margin of the carapace, and the rounded posterior 

 extremity of the axis ; a caudal shield being present. 



And I shall distinguish each specimen by a number instead of 

 a name. 



Group A. No. i. 



The body is broadly ovate, tapering slightly towards the caudal 

 extremity; the entire length is eight-tenths, and the width six-tenths 

 of an inch ; axis, narrow ; pleura about one-half wider than the 

 axis. Cephalic shield short and broad, the glabeUa expands 

 slightly towards the front margin, which is well rounded, and the 

 upper, middle, and basal lobes are well developed. The cheeks are 

 ornamented with lateral ridges and furrows, in a line with the lobes 

 and furrows of the glabella; axal furrows clearly defined, eyes 

 indistinct. The axis expands slightly from the posterior margin of 

 the glabella to the middle of the thorax, and then tapers gently 

 towards the tail. There are nine thoracic segments left, but 



