AGNOSTUS CALVTJS. 23 



Head- and taR-shields, about 2'5 mm. long and wide. 



There can, I think, be little doubt that this form belongs to Linnarsson's 



species. The head agrees very closely with Linnarsson's figure. The tail differs 

 in the extreme shortness of the first two segments, which is, however, in part due 

 to compression. In Linnarsson's figure, moreover, the median tubercle runs 

 through the anterior and middle segments of the axis, and is clearly separated 

 from both. Tn our specimen the keel is distinctly a part of the middle segment, 

 and projects forwards between the lobes into which it divides the anterior segment. 

 So far, indeed, as the tail is concerned, Linnarsson's figure is much nearer to flic 

 following species, but the head is very different. From the evidence afforded by 

 our British specimens one would be disposed to infer that the head and fail of 

 Linnarsson's figure belong to different species. 



A. sidenbJadhi is closely allied to A. rudis and to .1. callavei; it is somewhat less 

 closely connected with A. dux; and in the structure of its tail-axis it even shows 

 affinities with the Lower Lingnla Flag form A. pisiformis var. obesus. From .1. nulls 

 it is distinguished by its much shorter tail-axis, and by the extreme faint ness of the 

 furrow which separates the anterior from the posterior lobe of glabella. From 

 .1. dux it is separated by the considerably greater length of its glabella and of the 

 axis of the tail. Its closest ally is undoubtedly A. callavei, F. Haw, which, in spite 

 of the difference in size, it resembles very closely in the proportions of its parts. 

 If, however, the remarkable diverging furrows in that form, which separate the 

 anterior lobe of the glabella from the posterior lobe, are not adventitious markings, 

 tins alone is a sufficient distinguishing feature. In A. callavei, moreover, there is 

 a, strong tubercle or ridge on the posterior lobe of the glabella. This is reduced in 

 A. sidenbladhi to an almost indistinguishable prominence. 



Il<<ri::<ni and Locality. — Tremadoc: Penmorfa (collected by Mr. W. (I. Fearn- 

 sides). 



20. Agnostus calvus, sp. nov. Plate II, fig. 18. 



This form is closely allied to the preceding, but occurs at a slightly lower 

 horizon. It differs chiefly in the following points. The glabella is somewhat 

 narrower and the anterior is distinctly separated from the posterior lobe. The 

 axis of the tail is longer, and the first two segments are proportionately much heller 

 developed; the anterior and second segments, which are nearly equal in length, 

 are both divided into three parts by longitudinal furrows, the central portion being 

 raised so as to form an elongated tubercle, on which the division between the two 

 segments is still visible; the posterior segment forms about half of the whole axis. 



Head- and tail-shields. 3 mm. long and wide. 



