﻿8G 



SILURIAN TRILOBITES. 



A much smaller species than the preceding, and distinct from it by abundant cha- 

 racters of shape and habit. The divergent spines of head, thorax, and tail, enable us 

 at once to recognise it ; and of the latter the remarkable extended first pair of pleurae 

 (the rest of the tail is lost) show a near connection with the JS. unicus, next described. 



Only two good specimens, 10 lines long, are known. The head is equal to the thorax 

 in length, and longer than the caudal portion. It has a very large globular front, longer 

 than the square stipes, and granular all over. This stalk or base seems to be only 

 furrowed beneath. The cheeks are granular, gibbous, with a prominent eye on the front 

 edge near the glabella; and directed forward, not outward, with a broad plain margin, 

 and widely divergent spines. 



A very finely preserved specimen, fig. 18, lately acquired 

 Fig- 18. by the Geological Survey, shows the body and tail-spines very 



well, and the free cheek furnished with short spines. 



The axis of the thorax is cylindrical, and as wide as the 

 stalk of the glabella. The pleurae flat as far as the fulcrum, 

 which is less remote than the width of the axis ; strongly tu- 

 berculate at this point, and thence with patent (not recurved) 

 spines, as long as the portion within the fulcra. The thorax 

 tapers backward rather rapidly to the tail, which has a short 

 three-ribbed axis; and the upper pair of its pleurae are 

 very much expanded widely divergent, and more arched than 

 in our figure, which also represents the thoracic pleurae as 

 s. ghbieeps s Portiock from Ayrshire. ] ess cur ved than thev really are. The hinder portion of 



Mus. P. Geology. * J 1 



the tail is absent on our specimen. 

 Locality. — Caradoc Rocks of Desertcreat, Tyrone ; also Ayrshire, — a solitary 

 specimen, figured in the woodcut, (Mus. Pract. Geology). 



A third form, very abnormal in its characters and of large size, was named in MS. 

 S. Maclareni by Prof. Wyville Thomson, after the veteran Scotch geologist in whose 

 company it was found. It is, however, Prof. Thomson's previously described Acidaspis 

 unica. As he has mislaid his own full description, I may supply the following notes 

 from his specimens, and others presented to the Museum of Practical Geology by himself. 



S. ? unicus, Wyv. Tliomson. PI. VII, figs. 22—24. 



Acidaspis unica, Thomson. Quart. Geol. Journ., vol. xiii, pi. vi, fig. 13; 1857. 

 Staurocephalus ? unicus, Salter. Decade 11 ; Geol. Surv., Art. 5, 1865. 



S.\\-uncialis, oblongus, sparse granulosus, glabella gibbd eminentissimd, corpore piano, 

 caudd expansd transversa. Caput latum, glabella clavafd elevatd frontem longe impendente, 

 d genis punctalis distinctissimd ; margine crasso utrinque bispinoso. Pleura subplance, 



