ZONIDISCUS? TRANSIENS. 73 



2. Zonidiscus grayi, var. shallochensis, nov. Plate XI, figs. 16, 17. 



Varietal Characters. — -Shell much compressed, lenticular, dorsally acute, 

 composed of about 3 whorls, thickest on umbilical edge. Whorls high, narrow, 

 triangular ; dorsum carinated, narrow, acute ; sides of whorls gently convex to 

 flattened, somewhat swollen at base, with narrowly rounded or subangular umbilical 

 edge ; umbilical slope short, vertical. Umbilicus open, about one-fifth the diameter 

 of the shell and with centre situated at about two-fifths its height. Mouth not 

 expanded, with narrow median open slit in outer lip, followed by narrow slit-band 

 (elevated on carina) with fine raised edges and strong distant lunula?. Surface of 

 shell ornamented by fine transverse regular sublamellose equidistant lines running 

 obliquely back from umbilical edge to meet dorsal edge at about 75°, but scarcely 

 curved. 



Dimensions. — Height of shell, 20 — 27 mm. 



Horizon. — Whitehouse Group, Middle Ordovician. 



Locality. — Shalloch Mill, Girvan. 



Remarks. — The type-specimens of this variety are in Mrs. Gray's Collection. 

 The difference between this form and '/. grayi lies in the more rapid increase 

 in height of the outer whorl, the smaller umbilicus and the ornamentation, the 

 lamellas being thicker and coarser ; but they are undoubtedly very much alike. 

 Unfortunately the mouth and slit are not well preserved in any of the Shalloch 

 Mill specimens. In Mrs. Gray's list (p. 696) in the 'Survey Memoir' this variety is 

 entered with Z. grayi&s B. carinatus, Sow. 



3. Zonidiscus? transiens, sp. nov. Plate XI, figs. 18 — 21. 



Specific Characters. — Shell lenticular, compressed, thickest at umbilicus, com- 

 pletely involute, composed of few whorls ; outer whorl very slowly increasing in 

 size, tectiform, subtriangular, as high as wide or rather higher than wide, com- 

 pletely embracing inner whorls ; dorsum narrow, flattened, occupied by flat or 

 slightly depressed slit-band ; sides of whorl gently convex or somewhat flattened, 

 rarely a little swollen at base. Umbilicus very small, deep, central, with sub- 

 angular umbilical edges. Mouth high, subtriangular, not expanded ; upper lip 

 with deep long narrow V-shaped sinus followed by long slit ; apertural lobes 

 rounded, large. Surface of shell ornamented with very fine closely set, slightly 

 wavy, and somewhat interrupted revolving lines parallel to slit-band, becoming 

 faint or obsolete near umbilicus, and crossed by fainter gently-arched delicate 

 transverse lines, distinct near umbilicus, but scarcely visible near periphery. 



Dimensions. — Height of shell, 10 — 12 mm. ; thickness at umbilicus, 5 — 6 mm. 



Horizon. — Balclatchie Group, Lower Ordovician. 



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