AMPYX MACALLUMI. 21 



without a fulcrum. Its size is rather great, being 10 mm. in length, but not too 

 great for some of the head -shields of A. hornei. 



Collections. — Mrs. Gray (f. M.) ; Museum of Practical Geology (f. M.) ; Edinburgh 

 Museum; Woodwardian Museum; Hunterian Museum. 



Horizon and Localities. — Balclatchie Group (Llandeilo) : Balclatchie ; Ardmillan ; 

 Penwhapple Glen. 



4. Ampyx macallumi, Salter, 1865. Plate III, figs. 11, 12. 



1865. Ampyx macallumi, Salter, MS., Cat. Foss. Mus. Pract. Geol., p. 18. 



1879. Ampyx macallumi, Nicholson and Etheridge, Mon. Silur. Foss. Girvan, fasc. ii, p. 180, pi. xii, 



figs. 9—12. 

 1899. Ampyx macallumi, Mem. Geol. Surv., Silur. Rocks Brit., vol. i, Scotland, pp. 509, 514, 672, etc. 



Remarks. — Only the head-shield and pygidium of this species were described 

 and figured by Nicholson and Etheridge (M., fasc. ii, 1879, p. 180, pi. xiii, 

 f. 9 — 12), but some complete individuals from Dow Hill belonging to the same 

 species render the description of the thorax now possible. Some corrections with 

 regard to the description of the pygidium are also necessary. 



In the case of the head-shield it may be remarked that there are indications of 

 two small nodules on the base of the glabella in some of the Balclatchie specimens, 

 but especially clear in some of the Dow Hill examples. A deep elongated pit in 

 each axial furrow near the middle of the glabella is also noticeable in some. The 

 frontal spine is apparently grooved as in .1. rostratus. The genal spines probably 

 belonging to this species are described by Nicholson and Etheridge from 

 Balclatchie (M., loc. cit., p. 182), but in some nearly complete individuals from 

 Dow Hill similar spines are seen actually attached to the head-shield and are 

 about twice the length of the thorax. Similar spines are found in some specimens 

 from Ardmillan. 



The thorax is about half the length of the glabella (without the frontal spine), 

 and consists in the specimens which I have examined of only five segments. It is 

 widest across the middle, the first segment having shorter pleurae with very 

 obliquely truncate ends. The axis is one third the width of the thorax and 

 gently convex. The pleurae are straight, horizontally extended, and in contacl 

 with the surface marked by a wide groove deepening towards the extremity, 

 which is abruptly truncated. The first segment is rather larger than the others. 



The pygidium is erroneously said to be semicircular; in the type specimen 

 from Balclatchie as well as in all the others from that locality ami from Dow Hill 

 it is subtriangular, pointed posteriorly, with the sides rather sigmoidal in outline ; 

 and the width is twice as great as the length. The axis is broad, being nearly one 

 third the width of the pygidium, and tapers rather rapidly to the extremity ; it 



