BRONTEOPSIS ARDMILLANENSIS. 93 



groove ending at fulcrum which is weak, and situated about halfway out or rather 

 more. 



Dimensions. — 



Length of head . . . 8"5 mm. 



Width of „ . . . 23-0 „ 



Affinities. — The general characters of the head-shield of this species ally it closely 

 to B. scotica, Salter (q.v.), but it differs in the absence of paired pits on the glabella, 

 the presence of basal lobes to the glabella and of the tubercles on the neck-ring, 

 the narrower neck of the glabella and the course of the facial sutures. With 

 Ogygia? concentrica, Linnarsson, 1 it agrees in the position of the eyes, course of the 

 facial suture, tubercle on neck-ring and surface-contour of the cheeks, but there 

 are no pits on the glabella, and the genal angles are less produced. Except 

 for the want of glabellar pits and the presence of the tubercles on the neck-ring, 

 the head-shield of Br. ardmillanensis closely resembles that of Bronteus hi mil us, 

 Billings, 2 from the Trenton Limestone. 



Remarks. — There are two head-shields belonging to this species of Bronteopsis 

 in Mrs. Gray's collection from Ardmillan, and another from Balclatchie, one of 

 which is nearly perfect and has its external cast also preserved. There are some 

 poorly preserved pygidia associated by Mrs. Gray with this form, and from the 

 same locality (Ardmillan), but they have not so far been found directly attached to 

 the thorax or connected with the head-shield. In the Museum of Practical Geology 

 there are three with the same characters, also from Ardmillan, labelled Stygina 

 latifrons. 3 It is probable that all these pygidia are referable to Bronteopsis 

 ardmillanensis, as they are of the proper relative size and are in many respects 

 similar to those of the allied species B. scotica. Their shape is semicircular to 

 semielliptical, generally twice as broad as long, with a distinct raised fulcrum 

 situated about halfway out on the anterior margin with a groove behind it. Round 

 the margin of the pygidium is a broad shallow excavation. The axis is narrow 

 and subcylindrical, being in width about one fifth to one fourth that of the 

 pygidium, and extending from two thirds to three fourths its length; it bears 8 — 9 

 rings, generally faint, and has a narrow terminal pointed appendix or ridge running 

 back to the margin. On the lateral lobes are faint curved pleurae. It is deserving 

 of notice that some of the pygidia attributed by Winian ' to Stygina latifrons 



1 Linnarsson, 'Vestergotl. Camb. Silur. Aflagr.' (18691, p. 75, pi. ii, figs. 37—40. 



2 Billings, ' Geol. Surv. Canada, Rep. Progr.,' 1853-56, p. 338; Clarke, 'Geol. Surv. Minn.,' vol. iii, 

 pt. 2 (1894), p. 725, fig. 43; Ruedemann, 'Bull. New York State Mus.,' No. 49 (1901), p. 65, pi. iv, 

 figs. 10, 11. 



3 Armstrong ami Young also record Stygina latifrons from this locality, ' Cat. West. Scot. Foss.,' 

 p. 16. 



4 Wiman, 'Bull. Geol. Instit. TJpsala,' No. 10, vol. v, pt. 2 (1900), p. 171, pi. v, fi^s. 17, 19 

 (non Bg. 16). 



