MICRODISCUS SPECIOSFS. 33 



specimens. Hall, and Shaler and Foerste represent the glabella as reaching to 

 the anterior margin, which in Shaler and Foerste's figure is much broader in front 

 of the glabella than at the sides (as in the Comley form). In Walcott's figures the 

 glabella does not reach the margin, and the margin is not expanded in front. In 

 one of Hall's figures the glabella is cylindrical, in another it is narrowest in front; 

 in Shaler and Foerste's figure it is constricted in the middle ; in one of Walcott's 

 figures it is conical, in the other it is a little narrowed at the middle lobe. The 

 occipital furrow appears always to be strong, but the fainter transverse furrows 

 are only occasionally seen — the greater number of examples, according to Walcott, 

 are without them. Walcott remarks on the variability in the outline of the head 

 and in the form of the glabella. 



Horizon and Locality. — Cambrian : limestone beds 200 yards south of Comley 

 Quarry, Shropshire. In America the species belongs to the Olenellus zone. 



2. Microdiscus speciosus, Ford. Plate III, fig. 7. 



1873. Microdiscus speciosus, Ford, Amer. Journ. Sci., ser. 3, vol. vi, p. 137, fig. 2 a, b. 



1877. Microdiscus speciosus, Ford, loc. cit., vol. xiii, p. 141. 



188(3. Microdiscus speciosus, Walcott, Bull. U. S. G-eol. Surv., uo. 30, p. 154, pi. xvi, figs. 3, 3 a — c. 



1890. Microdiscus speciosus, Walcott, Tenth Ami. Rep. U. S. Geol. Surv., \>. G3'2, pi lxxxi, figs 5,5a — c. 



1896. Microdiscus specioszis, Matthew, Trans. N. Y. Acad. Sci., vol. xv, p. 236, pi. xvii, fig. 6. 



Head convex, semi-elliptical, somewhat acute in front, and obliquely rounded 

 at the genal angles, with a narrow margin. Glabella conical, forming about one 

 third of the whole width, elevated above the cheeks, and separated from them by 

 distinct but not very deep axial furrows, reaching forward nearly to the anterior 

 margin, and projecting slightly backwards beyond the posterior margin, without 

 any sign of occipital or glabellar furrows. Cheeks sloping downwards from the 

 glabella to the margin, wide posteriorly, narrowing forwards, confluent in front of 

 the glabella, surrounded posteriorly as well as round the sides and front by a 

 marginal groove and rim. Margin narrow, slightly expanded in front, consisting 

 of a groove and an external raised rim, both of which are continued behind the 

 cheeks to the glabella, with six or seven distant tubercles upon the rim on each 

 side, and one behind the cheeks. 



Head-shield 8 mm. long, 1) mm. wide. 



The specimen here described, the only one yet known in Britain, was found, 

 with M. Inbuilt.*, by Mr. E. S. Cobbold in an exposure of limestone 200 yards south 

 of Comley Quarry. It agrees in every particular with the figure and description 

 given by Ford of the head of M. speciosus, and it closely resembles Walcott's fig. 

 3 c in ' Bull. U. S. Geol. Surv.,' no. 30, pi. xvi, although in that figure the glabella 



