LOWER PALAEOZOIC HYOLITHID^ FROM GIRVAN. 207 



Horizon and Locality. — Balclatchie Group, Balelatchie. 



Remarks. — This species is allied to H. immemor (q.v.), from the same horizon, but 

 differs by its more slowly tapering form, the non-chambered apex (so far as known), 

 the more strongly angulated ventral face, and the sharp lateral angles. The longi- 

 tudinal ornamentation of the ventral face is only very faintly seen in one specimen, and 

 its existence is, indeed, open to doubt ; it is at any rate different to that of H. immemor. 



H. simplex, Salter,* is allied, but has not a sharply angulated ventral face, and 

 tapers more rapidly ; the operculum referred to this species by Salter bears a con- 

 siderable resemblance to that here mentioned under H. immemor, and the species 

 occurs in the Lower Llandeilo of Shelve. H. vaginula, Salter, f is a shorter, broader 

 shell, but has an angulated ventral face. 



H. triangularis (Portl.),| from Tyrone, is a larger shell, one of the type-specimens 

 measuring just 40 mm. in length, but its rate of tapering and shape of cross-section 

 seem identical ; the dorsal (flat) side is, however, stated to be longitudinally striated, 

 while the ventral face has only transverse striae ; but all the specimens are very badly 

 preserved, and the lineation, which is very unsatisfactorily seen, is at any rate coarser. 



Hijolithes immemor, sp. nov. (PI. I. figs. 18-21. ; PL II. fig. 1.) 



Shell straight, tapering at about 1 in 3|-, with the apical portion (i.e. the posterior 

 fourth part of the shell) tapering more gradually and divided up by transverse 

 partitions into 12-15 chambers. Dorsal face gently convex, with traces of narrow 

 border. Dorsal lip gently arched. Lateral angles acute, but not sharp. Ventral face 

 angulated, with strong submarginal raised longitudinal rib on each side, and two median 

 weaker ribs along the central keel, but rest of surface smooth (imperfectly known). 

 Surface of dorsal face marked by fine transverse growth-strise, with traces of 1-2 

 marginal longitudinal lines occasionally visible. 



Dimensions : — 



Length . . . . . . . .21-0 mm. 



Width of mouth . . . . . . 5-0 „ 



Horizon and Locality. — Balclatchie Group, Ardmillan. 



Remarks. — This small shell has very constant proportions, but in no case is the 

 ventral face well seen ; the marginal groove and border on the dorsal face are only 

 visible when the shell is preserved, and then generally only in the apical half. The 

 chambering of the apex and the slender, more tapering shape of this portion are 

 remarkable, and have not been observed in other species from Girvan. The shell 

 tapers less rapidly than H. ardmillanensis, but is probably closely allied to it, and 

 imperfect casts are difficult to distinguish. Its relations to H. girvanensis have been 



* Salter, Mem. Geol. Surv., vol. iii., 1866, p. 352, pi. xi.B. figs. 22-26. 



t Ibid., p. 352, pi. x. fig. 14. 



I Portlock, Geol. Rep. Londond., p. 375, pi. xxviii.A. figs. 3 a-c. 



