30 BRITISH CAMBRIAN TRILOBITES. 



median tubercle. Lateral lobes very slightly narrowed posteriorly, and separated 

 behind the axis by a furrow. 



Head- and tail-shields 5 — 6 mm. long and wide. 



The heads are, unfortunately, not complete, and do not show the hinder 

 portion. According to Tullberg, the posterior lobe of the glabella bears a median 

 tubercle, and the basal lobes are triangular and confluent behind. In B rugger's 

 and Tullberg's figures the anterior lobe is represented as much more acute 

 in front than in our specimens ; but this is probably owing to the somewhat 

 imperfect preservation of the British specimens, and the fact that they are 

 without the test. The tail agrees exactly with the Scandinavian form. 



Amongst British species the nearest ally is A. exaratus, Gronwall. In that 

 species, however, the axial furrows are much deeper, both in the head and in the 

 tail. The axis of the tail, moreover, although it is produced into a depressed 

 point, is not so suddenly constricted as in A. incertus, nor is the pointed termina- 

 tion so long. The segmentation of the axis is even less distinct than in the latter 

 species. 



Horizon and Locality. — Cambrian: from "supplementary stream seven chains 

 south-west of Neves Castle" [Shropshire] "four yards below Pa rad oxides Bed, 

 irregular brecciated bed in crushed shales." In Scandinavia this species occurs in 

 the Paradoxides zone. 



Genus MICRODISCUS, Salter (non Emmons). 



(? Pemphigaspis, Hall.) 



The genus Microdiscus was first established by Emmons ] in 1855 for a small 

 trilobite, to which he gave the name M. quadricostatus. In 1864 Salter 2 referred 

 to the same genus the British form which has ever since been known as 

 M. punctatus; and, in practice, it is Salter's species which is now universally 

 taken as the type. Unfortunately, however, it does not belong to the same genus 

 as Emmons's specimen, and consequently the name proposed by Emmons has 

 entirely lost its original significance. As long ago as 18G1 it was pointed out by 

 Barrande 3 that M. quadricostatus resembles the young of Trinucleus, and that 

 Emmons was probably in error in considering it more ancient than the rest of his 

 Taconic trilobites. Walcott 4 has since made a similar observation, and states that 



1 American Geology, vol. i, pt. 2 (1855), p. 116, pi. i, fig. 8. 



2 Quart. Journ. Geol. Soc, vol. xx (1864), p. 237. 



3 Bull. Soc. Geol. France, ser. 2, vol. xviii (1861), p. 280, pi. v, fig. 13. Barrande reproduces 

 Emmons's figure and description. 



4 Bull. TJ. S. Geol. Surv., no. 30 (1886), p. 152. 



