THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 509 



in surface ornament that I have no hesitation in erecting it into a new species. None 

 of the many species of the genera which Ulrich has described from the Lower Silurian 

 rocks of Ohio have the subcircular shape of the species under description. Fortunately, 

 a specimen from Newlands is in the form of an internal cast, which shows all details of 

 the interior except the cardinal region of the hinge plate. 



Genus Paracyclas, Hall, 1843. 



Paracyclas, Hall, 1843, Geol. Surv. N. York Rep., 4th Dist., p. 171. 

 Lucina, Portlock, 1843, Rep. Geol. Londonderry, p. 571. 

 ,, M'Coy, 1844, Synops. Carb. Foss. Ireland, p. 53. 

 Paracyclas, Hall, 1885, Geol. Surv. N. York, Pal., vol. v., pt. i., Lamell., ii., p. xxxviii. 



,, Beushausen, 1895, Die Lamell. rheinisch. Devon., p. 165; Abh. kimigl. preuss. Geol. 



Landes., n.f., pt. xvii. 



Generic Characters. — Shell equivalve, subequilateral, suborbicular or broadly sub- 

 elliptical. Anterior end regularly rounded, posterior end rounded or subtruncate, some- 

 what more produced below than the anterior. Beaks small and low, generally rising 

 little above the hinge line. Hinge line short. Post-cardinal slope more or less defined 

 by an oblique furrow or depression, which sometimes leaves the extremity subalate. 

 Surface concentrically striate, sometimes with strong concentric ridges marking the 

 exterior. Structure of hinge not fully observed. Ligament supported on each side 

 internally by a narrow plate, and leaving in the cast two narrow grooves directed 

 forward from the beak. Muscular impression in the post-umbonal slope. Pallial line 

 parallel with, and a little within, the margin of the shell. 



Observations. — The above is Hall's full diagnosis in 1885 [op. supra cit.). The 

 shells that I now refer to the genus are smaller than those species which are known in 

 the Devonian or Carboniferous rocks. Unfortunately, I have gained no definite know- 

 ledge of the hinge plate in the older species. 



Paracyclas minor, sp. nov. (PL IV., figs. 32-35.) 



Specific Characters. — Shell small, suborbicular, very moderately gibbose. Anterior 

 end rounded. Inferior border almost semicircular. The posterior subtruncate. The 

 hinge line arched. The umbones small, subcentral, slightly raised, obtuse. The dorsal 

 slope is compressed. 



Interior. — Not exposed. 



Exterior. — The shell is ornamented with regular, fine, close, concentric lines, often 

 separated into groups by deeper concentric linear grooves, especially near the lower 

 margin. 



Dimensions. — Fig. 32, PI. III., measures: antero-posteriorly, 10 mm.; dorso- 

 ventrally, 10 mm. ; elevation of valve, 8 mm. 



Localities. — Shalloch Mill, Middle Bala ; Thraive and the Star-fish bed, Upper Bala ; 

 Woodland Point, Middle Llandovery. 



