AGNOSTUS TARDUS. 7 



the middle by narrow longitudinal ridge dying out on posterior segment. Limb 

 ornamented by radiating bifurcating ridges similar to head-shield, but no distinct 

 parallel simple pair behind axis. 



Remarks. — There was some doubt in the minds of Professor Nicholson and 

 Mr. Etheridge whether the Grirvan specimens really belonged to Barrande's 

 species, A. perrugatus, but a minute examination of a series of specimens, including 

 the types, from Whitehouse Bay removes any hesitation in definitely referring 

 them to the Bohemian species. 



Collections. — Mrs. Gray (f. M.) ; Museum of Practical Geology, Jermyn Street ; 

 Edinburgh Museum. 



Horizon and Locality. — Whitehouse Group (M. Bala) : Whitehouse Bay. 



1. Agnostus tardus, Barrande, 184(3. Plate I, figs. 8, 9. 



1846. Battus tardus, Barrande, Not. prelim., p. 35. 



1847. Arthrorhachis tarda, Conla, Prodr. Bohm. Trilob., p. 115, pi. vi, fig. GO. 

 1852. Agnostus tardus, Barrande, Syst. Silur. Bolieme, vol. i, p. 913, pi. xlix. 



Specific Characters. — Body strongly swollen transversely; shape oblong, 

 rounded. 



Head-shield semi-oval (there is a long form and a short form). Glabella semi- 

 cylindrical to semi-oval, one fourth the width and rather less than one half the 

 length of the head, regularly convex, without lobes. Axial furrows strong, and 

 in them at base of glabella on each side is a small tubercle. Limb (genal zone) 

 of regular width. Border a little wider in front than behind, with well-marked 

 marginal groove and small genal spines at posterior lateral angles. 



Thorax of two segments ; axis wide, two thirds the breadth of thorax, showing 

 median and two lateral nodules. Pleurae very short and grooved. 



Pygidium with semi-oval axis a little less than half the width and half the 

 length of pygidium, and showing traces of three segments ; anterior segment like 

 thoracic segment, with three nodules and marked by two small longitudinal furrows ; 

 posterior segment rudimentary. Border simple, well developed, like that round 

 head-shield, without lateral spines. Surface of body smooth. 



Remarks. — The above description is that of Barrande, based on the Bohemian 

 specimens. Only one example of the head-shield and one of the pygidium of this 

 species from the Girvan district have so far come under my notice. Both are from 

 Whitehouse Bay, and show the characteristic features of the species. 



Affinities. — This species is so similar to A. agnostiformis that it requires a very 

 close examination to detect the differences, particularly in imperfectly preserved 

 examples. As Salter remarked (Mem. Geol. Surv., dec. xi, 1864, art. 1, p. '.»). 

 A. tardus has no trace of the slight lateral indentations on the sides of the glabella, 



