I. 



II. 



26-0 



c. 23 - mm, 



10-5 



c. 9-0 „ 



204 MR F. R. COWPER REED ON 



Dimensions :— 



Length ...... 



Width at mouth ..... 



Horizon and Locality. — Balclatchie Group, Ardmillan. 



Remarks. — This is a well-characterised form. Generally only the dorsal face is 

 preserved or the internal cast of the shell. The strong, regular longitudinal lineation 

 of the ventral face and the short, broad shape of the shell are distinguishing features. 



Holm's H. cymbium* from the Lituites Limestone, appears to be closely allied in 

 both these features, but has finer longitudinal lines and a slightly curved shell. 



The species described by REMELE + as H. incequistriatus, from the boulders of 

 Orthoceras Limestone in the North German Drift, closely agrees with our species in 

 shape and rate of tapering, and the ornamentation is of the same type, though the 

 longitudinal lines vary in strength. 



Some unnamed specimens from the Bala beds of Horderley, in the Jermyn Street 

 Museum, bear considerable resemblance to H. ardmillanensis in cross-section, rate of 

 tapering, and ornamentation, but are too poorly preserved to warrant identification 

 with it. 



An operculum (PI. I. fig. 6) occurs associated with the above shells at Ardmillan 

 which is of the requisite proportions and may probably be referred to this species. 

 It is subelliptical in shape, folded sharply along a line near the ventral margin, 

 parallel to the longer diameter, and is conical and moderately elevated, with the apex 

 situated on the fold close to the ventral margin. The ventral side is short, narrow, 

 very steeply inclined, and almost at right angles to the rest of the surface. The 

 general surface of the cone, though rising to the apex, is somewhat concave, and is 

 marked with three strong radiating grooves on each side, diverging from the apex, and 

 not reaching the margin. As we are dealing with an internal cast, these grooves 

 really represent internal ridges in the operculum, as Barrande pointed out. 



Fine radiating lines occur on each side of the sharp, angular junction of the 

 ventral side with the general surface, and are traceable (though less distinctly) near the 

 margin on other parts of the surface, but over the greater part only very delicate 

 concentric striae can be detected. 



The transverse diameter of one such operculum is 11*5 mm. 



Barrande | has figured from Stage Ddl a somewhat similar isolated operculum 

 with the internal cast showing three radiating grooves on each side ; these he interprets 

 as muscular impressions. 



It should be noted that none of the Girvan opercula have been found in position, 

 but their relations to the shell are inferred from the Bohemian species described by 



* Holm, "Sver. Kamb.-silur. Hyol. o. Conul." (Sver. Geol. UndersoJcn., Ser. C, No. 112, 1893), p. 82, t. iii. 

 figs. Oil. 



+ Kkmixi', Z,;t,,hr. dent. geol. Gesell, Bd. xl., 1888, p. 670, t. xxviii. figs. 4-6. 

 J BaBEAOTJE, Sijst. SiJm: Boheme, vol. iii., 1867, p. 98, pi. ix. figs. 16, 17. 



