522 DR WHEELTON HIND ON 



Ctenodonta dissimilis, Portlock, sp., 1843. (PI. III., figs. 5-7.) 

 Area dissimilis, Portlock, 1843, Rep. Genl. Londonderry, p. 428, pi. xxxiv. fig. 5. 



Specific Cliaracters. — Shell below medium size, moderately compressed, inequi- 

 lateral, obliquely oval. The anterior end is narrowed, chiefly at the expense of its 

 lower margin ; its border bluntly rounded. The inferior border convex ; the posterior 

 bluntly rounded. The hinge line very gently arcuate. The umbones pointed, 

 raised, incurved, and hollowed out in front ; placed in front of the middle line. The 

 umbonal region is moderately gibbose, but the valve is rapidly compressed towards 

 its margin. 



Interior. — The hinge plate is normal. 



Exterior. — The surface is ornamented by fine concentric lines. 



Locality. — Drummuck, Thraive Glen, Upper Bala. 



Dimensions. — Fig. 7, PI. III., measures : antero-posteriorly, 9 mm. ; dorso-ventrally, 

 7 mm. 



Observations. — After comparison with Portlock's type, I have referred four 

 specimens to his species provisionally. There is doubt, from Portlock's description 

 of the hinge plate, that the species belongs to Ctenodonta rather than to Area, although 

 he says " the species approximates in the arrangement of the teeth to the genus Area." 

 Only four specimens have been obtained in Scotland. 



Ctenodonta lingualis, Phillips, sp., 1848. (PI. III., figs. 8, 9.) 

 Nucula lingualis, Phillips, 1848, Mem. Geol. Surv. Great Britain, vol. ii., pt. i., p. 367, pi. xxii. fig. 6. 



Specific Characters. — Shell transversely ovate, postero-inferior angle produced, 

 gibbose, inequilateral. The anterior border rounded, the inferior broadly convex. The 

 posterior obliquely truncate, almost straight. Hinge line arcuate, erect posteriorly. The 

 umbones well developed, gibbose, incurved, and twisted forwards, placed at the 

 junction of the anterior and middle thirds of the hinge line. The dorsal slope is com- 

 pressed and hollowed. No lunule or escutcheon. 



Interior. — Normal. 



Exterior.— The surface is ornamented with fine concentric lines and rugas of 

 growth. 



Dimensions. — Fig. 8, PI. III., a bivalve example, measures: antero-posteriorly, 

 21 mm. ; dorso-ventrally, 8 mm. ; from side to side, 9 mm. 



Localities. — Drummuck, and Middle Bala of Shalloch Mill. England : Lower 

 Caradoc beds of Eastnor Park. 



Observations. — Three specimens in the collection I refer to Phillips' species, and 

 they differ markedly from any other forms of the same genus in general characters. 

 The transversely oval shape, erect hinge line posteriorly, and the hollow dorsal slope 

 at once distinguish the species from all others. 



