﻿106 



SILURIAN TRILOBITES. 



Glabella without lobes. These general characters will enable us to recognise the species, 

 which is a very common one in the Caradoc sandstones and slates of Britain, and 

 especially of Shropshire and North Wales. The finest head is from Mr. Lightbody's 

 cabinet (fig. 9) ; the most complete body and tail are from the Geological Society's 

 Museum (fig. 4). The species is a large one, and, judging from fragments, must have 

 been fully ten inches long. It is ovate and blunt at both extremities. 



Head semioval, of which the glabella occupies three fifths ; it is pyramidal, but with 

 sinuous sides, and truncate above ; and at its base is exactly as broad as the cheek. 

 It is rather abruptly raised above these, in a step-like manner. The cheek itself is 

 gently convex, but not abruptly so towards the eye, which is placed about centrally 1 and 

 outside the highest point of the cheek, but does not nearly rise to the level of the glabella. 

 This gives a peculiar character to the present species, for in the genus Homalonotus the cheek 

 is often very convex. Young specimens (fig. 7) have the glabella greatly more distinct 

 and more separated from the cheeks than in the adult, and the proportions are not quite 

 the same, the glabella being longer. 



In front of the glabella the head is slightly concave only; and the cheeks are as gently 

 convex, without any central gibbosity or any strongly distinct margin. The neck-furrow 

 is distinct all along, especially in the cast (fig. 9). It is strongest beneath the glabella, 

 and then descends to a lower level beneath the cheek, along the base of which it is 

 continuous almost to the rounded angle. The facial suture is vertical in front of the eye, 



and beneath it curves boldly outwards. The 

 eye is small and apparently ovate (not globular, 

 as in the last species). The free cheek is tumid 

 just outside the eye, and much deflected. 



The body, of thirteen rings, has its axis sud- 

 denly and greatly wider than the base of the 

 glabella, as indicated by the diverging lines on 

 the neck-segment (woodcut). It is not actually 

 a great deal wider than the pleurae ; but, from 

 the general and regular convexity (the back is not 

 depressed as in H. delphinocephalus), it appears 

 much more on a front view. The axal line is 

 less strong in the adult than in younger speci- 

 mens (fig. 7), and is marked out chiefly by a 

 sudden angular thickening of the pleurae over 



E. bisulcatus, Marshbrook, Shropshire. . 



the fulcral point (see figs. 4, 5). (Inside the 

 crust this point is marked on the cast by an oblique furrow, indicating, of course, an 



Fig. 24. 



1 The deflected cheeks give the eye in our figure 9 too exterior a position ; and the eye is really placed 

 a little too far out in the figure. 



