4 BRITISH CAMBRIAN TRILOBITES. 



Thorax : axis raised on each side into a small knob, and the central portion 

 also slightly raised ; pleura) of the anterior segment divided by a groove into a 

 large anterior tubercle and a narrow posterior ridge ; in the posterior segment the 

 groove is more nearly central. 



Tail rounded, with a narrow margin. Axis divided into three segments ; the 

 front segment is a little wider than the rest, is very short, and is tuberculate at its 

 extremities ; the middle segment is hexagonal and is raised medially into a longi- 

 tudinal keel ; the posterior segment is triangular, anteriorly it is about the same 

 width as the middle segment, posteriorly it terminates in a blunt point. Lateral 

 lobes of nearly equal width throughout and confluent behind the axis. 



Head- and tail-shields, 3-3*5 mm. in length and breadth. 



The only other British species yet known in which the anterior lobe of the 

 glabella is cleft is A. reticulatus, which is at once distinguished by the reticulate 

 markings on the cheeks. 



The axis of the tail differs from that of most other allied forms in the fact that 

 it is not constricted at the second segment, so that the second and third segments 

 form a triangle with slightly convex sides. The line of demarcation between these 

 two segments is sometimes very indistinct (as in fig. I). 



The species varies considerably in the character of the furrows which occur 

 upon the cheeks and the lateral lobes of the tail. Sometimes these are rather faint 

 and sometimes they are very strongly marked, and occasionally they may be seen 

 on the glabella and the axis of the tail. Under the name trifissus Matthew has 

 separated a variety which is characterised by the presence of "two additional 

 furrows at the front of the glabella, one at each corner, beside the median furrow 

 that characterises the type." Gronwall also separates another variety under the 

 name perrugata, which is distinguished by two pairs of longitudinal furrows upon 

 the cheeks. 



In view of the great variation which is to be met with in the markings, these 

 varieties seem to be of slight value, and the British specimens suggest that those 

 furrows which are not radiate are sometimes the result of the crumpling of a test 

 which was originally soft. This view seems to be supported by the not uncommon 

 occurrence of longitudinal furrows on the glabella and the axis of the tail. 



Horizon and Localities. — Menevian : near Nine "Wells, St. David's; Dwrrhyd, 

 near Solva. 



'2. Agnostus punctuosus, Angclin. Plate I, figs. 4 — G. 



18-V2. Agnostus punrfuasus, Angclin, I'al. Scand., p. 8, pi. vi, fig. 11. 



1875. Agnostus punctuosus, Brogger, Greol. For. Stock. Forh., vol. ii, p. 576, pi. XXV, fig. 2. 



1879. Agnostus punctuosus, Brogger, Nyt Mag. for Naturv., vol. xxiv, p. 67, pi. vi, fig. 12 a, b. 



1879. Agnostus punctuosus (var. affinis), Brogger, ibid., p. 08, pi. v, fig. 2 a, b. 



