2 



BEITISH FOSSILS. 



It is instructive to find this, the most rudimentary form, asso- 

 ciated with other genera in which all the characters of the group 

 are fully developed, in the same primordial zone. But it is to be 

 noted, that either by excessive reduction, as in Agnostiis, or exces- 

 sive multiplication of segments, as in Paradoxides, the genera at 

 this early period exhibit a defective organization as compared with 

 those of later formations. There is no sort of equality between an 

 Agnostus or Olenus and a Phacops or Phillipsia. 



The figures we give of A. princeps possess more than an usual 

 interest, for they represent some of the oldest fossils known in N.Wales 

 which occur in the lower part of the " Lingula flags," considerably 

 below the characteristic Lingulce of the formation. They are in 

 countless numbers in the black slates near the waterfall of Felyn 

 Rhyd, near Maentwrog, and are there associated with a new Olenus, 

 and with rare specimens of Lingida. That they are characteristic 

 of the formation is evident from the fact that with the reappear- 

 ance of black earthy slate in the upper division of the formation, 

 the Agnostus reappears also in abundance, and our figure 3 is from 

 that part of the series. In S. Wales other and new genera accom- 

 pany the Agnostus in the lowest portions of the deposit. 



Description. — Not half an inch long, though frequently elon- 

 gated by pressure beyond that length. The general form of the 

 head and tail is about two-thirds of a, broad oval, truncated next 

 the thorax, pretty regularly convex, and strongly trilobed ; the 

 thorax joints do not together occupy one-third the length of the 

 head, and are narrower than its width. 



The head is a little longer than broad, smooth, with a narrow 

 distinct border. The glabella about as wide as the cheeks below, 

 but tapering forwards ; and divided, at more than two-thirds its 

 height, by a transverse furrow which separates it into two parts, 

 the lower oblong oval smooth, without lateral indentations, and 

 the upper a spherical- triangular lobe, from the end of which a 

 dividing line runs forward to the margin. A conspicuous pair of 

 triangular lobes lie at the base of the glabella, in part subtending 

 the convex cheeks, which are narrow below, broadest above, but 

 narrower again in front, where they are separated by the dividing 

 line. 



Thorax of two nodose joints, the anterior largest. Each is 

 strongly trilobate ; the pleurse convex, and with a strong groove 

 toward the tips, making them appear notched (fig. 1 a). The axis 

 too is trilobate, the central lobe very prominent, and strongly dis- 



