ISOSPIRA. 27 



subangular equidistant transverse ribs, becoming weaker on dorsum and near 

 suture line, set at right angles to dorsal carina, and separated by wider shallow 

 concave interspaces occupied by 10 — 15 fine concentric lines, not fimbriated but 

 cancellated by fine obliquely spiral lines. Dorsum very obtusely angulated by 

 weak low carina. Umbilicus large, wide, open, exposing all the whorls; centre 

 situated at about one-third the height of shell. Mouth vertical with whole 

 peristome somewhat thickened near edge and inner lip somewhat expanded and 

 reflexed. 



Dimensions. — Height of shell, about 30 lum. ; width of mouth, about 17 mm. 



Horizon. — Upper Ordovician: Drummuck Group (Starfish Bed). 



Locality. — Thraive Grlen, Girvan. 



IiemarJiS. — The shell named Bellerophon nodosus, Salter/ from the Teirw River, 

 near Llangollen, much resembles this species, but the whorls increase more rapidly 

 in size and have fewer and coarser ribs and fimbriated .lines in the interspaces. 

 Ci/rtolites orii.ntns, Conrad,^ has the sides angulated with an umbilical ridge at 

 which the ribs end, but is an allied species. 



Genus ISOSPIRA, Koken. 



Generic Gharncters. — Shell symmetrically coiled, with rapidly enlarging 

 rounded whorls ; growth-strise pass over dorsum without sinus or bending. No 

 keel. Mouth simple, entire. 



Koken's^ definition of this genus is accepted by Perner {op. cit., p. 88). It 

 appears to be allied to Gi/rtoliles, but we have not a full knowledge of its 

 characters, and its position amongst the Bellerophontacea is open to doubt. 



1. Isospira huttoni, sp. nov. Plate V, fig. 11. 



Specific Gharacters. — Shell composed of loosely coiled whorls, 1^ — 3 in 

 number, contiguous, not overlapping, rounded, subcircular in section, rapidly 

 increasing in size ; umbilicus situated at about one-third to one-fourth the height 

 of shell. Mouth not expanded, circular, without sinuation of lip. Surface 

 of whorls covered with numerous regular equidistant fine transverse lamellge, 

 minutely fimbriated, without sinuation except near beginning of outer whorl, 

 separated by rather wider interspaces crossed by faint spiral lines in continuation 

 of fimbriations on transverse lines. 



Dimensions. — Height of shell, about 14*0 mm. ; height of mouth, about 9'0 mm. 



' Salter, ' Mem. Geol. Surv. Gt. Brit.,' vol. iii, p. 555, text-fig. 15. 



2 Ulrich and Scofleld, op. cit., p. 860, pi. Ixii, figs. 27—29. 



3 Koken, ' Gastrop. Bait. Uutersilurs ' (1897), p. 137. 



