524 DR WHEELTON HIND ON 



am correct in referring them to this species ; but the shell is so devoid of any special 

 characters that for the present I think it best to refer it to C. transversa. The 

 specimens from the higher beds are larger, and equal the Irish specimen in size. 



Ctenodonta regidaris, Portlock, sp., 1843. (PL III., figs. 15-17.) 

 Area regularis, Portlock, 1843, Geol. Rep. Londonderry, p. 427, pi. xxxiv. fig. 2. 



Specific Characters. — Shell semicircularly oval, moderately but regularly convex, 

 almost equilateral. The anterior and inferior margins regularly curved, the posterior 

 slightly truncate. The hinge line almost straight. The umbones are central, raised and 

 pointed, small. The posterior slope slightly hollowed. 



Interior. — The hinge teeth are normal. 



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



Dimensions. — Portlock's type (holotype) measures: antero-posteriorly, 13 mm.; 

 dorso-ventrally, 9 mm. 



Localities. — Llandeilo beds at Dowhill, Craighead, anH Balclatchie. 



Observations. — The specimen figured by Portlock is a left valve, and is either 

 semi-decorticated or the cast of the exterior. Only one of Mrs Gray's specimens 

 shows the external characters, which are similar to those which obtained in Carboni- 

 ferous forms of the genus. 



Ctenodonta obliqua, Portlock, sp., 1843. (PI. III., figs. 18, 19.) 



Area obliqua, Portlock, 1843, Geol. Rep. Londonderry, p. 429, pi. xxxiv. fig. 6. 

 Ctenodonta obliqua, Murchison, 1859, Siluria, 3rd ed., p. 213, fig. 6. 



Specific Characters. — Shell suborbicular, equilateral , moderately gibbose, with 

 compressed dorsal slope. The anterior end is compressed, its margin rounded. 

 Inferior border broadly curved, meeting the posterior margin at an angle. Posterior 

 border oblique, straight, making a more or less obtuse angle with the hinge line, which 

 is straight and short. Umbones small, tumid, incurved, slightly raised, central. 

 Passing from the umbo to the postero-inferior angle is an oblique ridge separating the 

 hollowed dorsal slope from the rest of the valve. No lunule or escutcheon. 



Interior. — Hinge multidenticulate, normal. Adductor muscle scars normal. 



Exterior. — Shell covered with fine, somewhat irregular, close concentric lines of 

 growth ; towards the lower border, strong concentric sulci. 



Dimensions. — Fig. 18, PI. III., measures: antero-posteriorly, 15 mm.; dorso- 

 ventrally, 11 mm. ; from side to side, 5 mm. 



Locality. — Scotland : Upper Bala beds, Drummuck. Ireland : Desertcreat, Co. 

 Tyrone. 



Observations. — Portlock's type is the cast of the interior of the right valve. The 



