ILL^NUS yEMUH'S. 55 



the truncated lateral angles, and consists of two sets of rather remote stria 1 

 meeting at an obtuse angle in the middle line. 



This species differs from the common Girvan variety of the species (see below) 

 (1) in having an axis more clearly defined and with a more marked independent 

 convexity, (2) in the presence of the faint ribs on the lateral lobes, and (3) in 

 the greater relative width of the pygidiuin. 



Dimensions. — Length of pygidium, 20*0 mm. ; width of pygidium, 37"0 mm. 



Collection. — Mrs. Gray. 



Horizon ami Locality. — Penkill Group (Tarannon) : Penkill. 



la. Illsenus aemulus, Salter, var. Plate VITT, figs. G — 11. 



1879. Illsenus semulus?, Nicholson and Etheridge, Mon. Silur. Foss. Girvan, fase. ii, p. 157, pi. xi, 



fig-. 10. 

 1879. Illsenus nexilis ?, Nicholson and Ethericlge (e.p.), ibid., p. 158, pi. xi, fig. 13. 

 1899. Illsenus semulus, Mem. G-eol. Surv., Silur. Bocks Brit., vol. i, Scotland, pp. 536, 673. 



Remarks. — The common Llandovery form attributed to Illsenus semulus? by 

 Nicholson and Ethericlge differs somewhat from Salter's type of the species, which 

 was founded on a pygidiuin. The pygidium of this variety is of a more oval and 

 less transverse shape, the axis is less prominent, and there are no pleural indica- 

 tions on the lateral lobes, but the ornamentation of fine arched or angulated lines 

 is similar. Nicholson and Etheridge figured (op. cit.) an example of such a 

 pygidium, but apparently knew of no specimens attached to the thorax and head- 

 shield. There is, hoAvever, an entire individual amongst Mrs. Gray's new material, 

 with several isolated head-shields. The head has the general shape and appearance 

 of I. bowmani, but its whole surface is ornamented with numerous transverse fine 

 thread-like lines, slightly irregular and wavy, but for the most part concentric with 

 the anterior margin. The thorax consists of nine segments, and has an axis as 

 wide as the pleuras, but tapering slightly for the last four or five rings to the 

 pygidium. The pleurae seem to have their inner portion wider than in /. bowmani, 

 the fulcrum being further out ; the fulcrum is also rather less marked and less 

 angular, and the extra-fulcral portion less bent down. But these parts are in 

 a more or less imperfect state of preservation, though fragments are fairly 

 numerous. 



The epistome from Penkill figured by Nicholson and Etheridge as probably 

 belonging to /. nexilis may belong to this variety of /. aemulus. It has tin 1 anterior 

 border nearly straight and the posterior border strongly arched backwards, but 

 the width from side to side is nearly three times the length from back to front, 

 and thus it is utterly different from Salter's type of I, nexilis. The lateral 



