BRITISH FOSSILS. 



3 



Variations. — Some individuals (perhaps females) are much more 

 convex than others ; in some the margin is abruptly concave ; our 

 figured specimen is depressed. The number of lateral ribs to the tail 

 varies considerably, they do not seem to become more numerous by 

 age. The forks of the hypostome are long or short. The ornamental 

 lines which are so conspicuous in the external cast (PI. 24, Sil. Syst.) 

 exist, though less deeply sculptured, in all ; the internal cast, which is 

 more commonly found than perfect specimens, does not show them 

 distinctly. 



Affinities. — The specimen figured in Burmeister's work appears by 

 his description to have been from a " boulder of red limestone," and 

 hence it is very probably a worn specimen of A. heros (Dalman), from 

 the red limestone of Kinnekiille, Gothland. It is unfortunate that the 

 author should have transferred the sculpture of A. tyrannus to this 

 figure, for the axis is constricted in the middle and depressed, and the 

 lateral ribs long ; it is in fact a pretty good figure of A. heros, except in 

 the rounded tail ; in Swedish specimens the tail is pointed and the axis 

 percurrent. There is also a species in the lowest Silurian limestone of 

 New York so like ours, that we do not know how it is distinguished ; 

 there are, however, but seven or eight distinct ribs on the axis, and 

 those on the sides are said to be duplicate ; we refer to A. marginalis 

 (Hall), described in the Palseontol. New York. This is the more inter- 

 esting, as the Asajjlii of North America belong generally to the group 

 Isotelus.^ The species which most nearly resembles A. tyrannus, and 

 which I referred to it in Prof. Sedgwick's papers in the " Geological 

 Journal," is the A. Powisii, very common in the slates of North Wales. 

 The specimen of this latter species figured in the " Sil. Syst." is much 

 compressed, for the species is really more convex than that here 

 described, with deeper furrows on the pleurse, which are arched forward 

 at their ends ; and the tail axis is wide above, rapidly contracting in 

 width and becoming quite indistinct, its end only prominent ; the surface 

 seems smooth ; the sides of the tail have a very strong uppermost 

 furrow ; the remaining furrows are faint. Fine specimens of A. Powisii, 

 in the collections of the Geol. Survey and of Mr. Sharpe, show very 

 numerous lenses (about 7000) on each eye. 



History. — It appears not to have attracted the notice of Llhwyd, 

 although it is quite as common at Llandeilo as the Ogygia Buchii, ^jf 

 which he figured specimens ; nor does it seem to have been noticed 

 by any writer on trilobites previously to the publication of Sir R. 1. 

 Murchison's work. Emmerich, in his Dissertation, showed how close he 

 considered its affinities to be with Ogygia, by transferring it to that 



* The young animals of even this group, according to Halls figure, are trilobed and 

 ribbed in the tail ; but that can only be in a very young state. 



