LOWER PALEOZOIC HYOLITHID^ FROM GIRVAN. 211 



at about 1 in 3-4. The dorsal face is flat, with a strongly arched mouth. No shell or 

 ornamentation is preserved satisfactorily. One specimen measures 15 mm. in length 

 and 4 mm. in width at the mouth. 



Hyolithes, sp. ind. (7). 



A small species of Hyolithes occurs in the Craighead Limestone, but it is too poorly 

 preserved for specific determination. It is a broad, short form, tapering at about 1 in 3 ; 

 the dorsal face seems to be flat, with a very slightly arched mouth, but no further 

 features can be made out satisfactorily. The one specimen recognised measures about 

 7 mm. in length and 2*5 mm. in width at the mouth. 



Subgenus Orthoiheca, Novak, 1887. 

 Hyolithes (Orthotheca) subexcavatus, sp. nov. (PI. II. figs. 10-12.) 



Shell straight, slender, slowly tapering at about 1 in 8, with elongated, pointed 

 apical portion showing chambers. Cross-section subtriangular. Ventral face high, 

 sharply angulated, the sides meeting at nearly a right angle. Dorsal face concave, with 

 swollen margins. Lateral angles acute, but rounded. Mouth straight, truncate. 

 Ventral face ornamented with about 30 fine, straight, longitudinal thread-like lines, of 

 equal size and equidistant, crossed by very numerous finer, closely placed stria?, giving 

 the surface a cancellated appearance. Dorsal face with similar longitudinal lines and 

 transverse striae. 



Dimensions : — 



Length . . . . . . . 18"5 mm. 



Width at mouth . . . . . . 2"5 „ 



Horizon and Locality. — Balclatchie Group, Ardmillan. 



Remarks. — In shape this shell much resembles H. girvanensis, but differs by 

 tapering more slowly and by the absence of a dorsal lip and by the character of its 

 ornamentation. In none of the specimens is the surface of the dorsal face well 

 preserved, so that there is a little doubt as to the presence of the longitudinal 

 lineation upon it. The chambering of the apical portion recalls H. immemor. In 

 cross-section our shell resembles H. (0.) cor., Holm,* and H. (0.) excavatus, Holm,t 

 of the Cambrian, but the ornamentation is somewhat like H. (0.) fiuctuosus, Holm,! 

 from the Trinucleus Beds, and H. (0.) trinucleorum, Holm. § Our H. (0.) thraivensis 

 tapers more rapidly, but otherwise is closely allied. 



There is a fairly good example of the upper portion of a shell from Desertcreat, 

 Co. Tyrone, in the Jermyn Street Museum, which may be identified with this Girvan 

 species. The shape and ornamentation are identical, and the specimen, when perfect, 

 must have measured about 45 mm. long ; its width at the mouth is about 6 mm. 



* Holm, op. cit., p. 58, t. i. figs. 60-62. + Holm, op. cit, p. 61, t. v. figs. 67-71. 



I Holm, op. cit., p. 68, t. iii. figs. 53-57, t. v. figs. 88-91. § Holm, op. cit, p. 67, t. v. figs. 79-87. 



TRANS. ROY. SOC. EDIN., VOL. XLVII. PART II. (NO. 9). 31 



