22 TRILOBITES OF GIRVAN. 



has one distinct ring at the front end, but behind this it is trilobed by the presence 

 of two longitudinal rows of 7 — 8 small nodules. The lateral lobes are flat and have 

 5 — 6 pleurae slightly curved forwards indicated on them, the front ones being 

 most distinct. The limb is strongly bevelled and striated. 



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

 Edinburgh Museum ; Hunterian Museum ; Woodwardian Museum. 



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

 Dow Hill; Ardmillan. 



5. Ampyx mammillatus, Sars (e. p.) (?), 1835. Plate III, fig. 13. 



1835. Ampyx mammillatus, Sars (e.p.); Isis, pi. viii, fig. 4c (? f. 4a, non 46, 4d). 



1854. Ampyx mammillatus, Angelin, Pal. Scand., p. 80, pi. xl, fig. 3. 



1879. Ampyx mammillatus, Nicholson and Etheridge, Mon. Silur. Foss. Girvan, fasc. ii, p. 187. 



1893. Ampyx mammillatus, Vogdes, Anier. G-eol., p. 99. 



Specific Characters. — Glabella oval, rather pointed anteriorly, subcarinate, 

 equally convex and not swollen in front but furnished with a long straight 

 horizontal rounded spine, not grooved. Near middle of glabella is a pair of 

 isolated lateral pits, and behind them are two oblique furrows incompletely 

 separating off a pair of lateral longitudinal lobes with deep pits at their base. 

 Base of glabella slightly swollen transversely. Occipital ring well marked. 

 Fixed cheeks broad posteriorly, extend along whole length of glabella and are 

 united in front of it as in A. hornei, but do not possess any radiating lines on 

 their surface. A deep broad curved furrow separates off neck segment. 

 Dimensions. — 



Length of head (without spine) . . S'b mm. 



Width of ,, at base (without free cheeks) 8"0 ,, 

 Length of frontal spine . . . -f- 3*0 ,, 



Width of glabella across middle . . 2*5 „ 



Remarks. — A strict adherence to the definition of Ampyx hornei as regards the 

 characters of the glabella obliges us to separate this Girvan form, which, though 

 occurring on the same horizon and at the same locality and possessing much the 

 same general features, has a long frontal spine instead of a mere tubercle on the 

 frontal lobe of the glabella. Nicholson and Etheridge laid so much stress on the 

 existence of only a median tubercle instead of a spine in A. hornei that, unless we 

 consider this condition to be a mark only of sex or age and the types to have all 

 belonged to one sex or stage of development, there is no other course to follow. I 

 have, however, only seen one well-preserved specimen with the characters here 

 given. 



