512 DR WHEELTON HIND ON 



Dimensions. — Fig. 21, PI. IV., measures: antero-posteriorly, 14 mm.; dorso-ventrally, 

 10 mm. 



Locality. — Upper Llandovery at Penkhill. 



Observations. — This specimen is provisionally referred to the genus Cyrtodonta, 

 though no details of the hinge plate are to be seen. It is much more modioliform and 

 has its posterior end more expanded than C quadrata, Salter, sp., with the type of 

 which I have been able to compare it. The specimen consists of a slab with both 

 valves lying open, of which the right has lost its lower border. 



Cyrtodonta gibbosa, Salter. (PI. IV., fig. 17.) 



Specific Characters.— Shell of medium size, oblique, gibbose, subquadrate, inequi- 

 lateral. The anterior end is compressed, its border rounded. The inferior border is 

 strongly convex. The posterior, oblique, truncate, almost straight. The hinge line 

 straight, equalling in length the longest antero-posterior diameter of the shell. Postero- 

 superior angle obtuse. The umbones are swollen, incurved, and pointed, and placed at 

 the junction of the anterior and middle thirds of the hinge line. The body of the shell 

 is strongly convex, the dorsal slope much compressed. Anteriorly there is a small 

 oblique compression. 



Interior. — The hinge plate consists of two anterior oblique teeth separated by 

 grooves. Posteriorly, two long oblique teeth, which become thickened as they pass 

 backwards. 



The anterior adductor muscle scar well marked, round, and deeply placed within the 

 umbonal hollow, remote from the margins ; the posterior shallow and inconspicuous. 



Exterior. — The surface is ornamented with fine concentric lines of growth, some- 

 what flexuous in front, and towards the inferior margin separated into grooves by a 

 deeper concentric groove. 



Dimensions. — Fig. 17. PI. IV., a left valve, measures: antero-posteriorly, 28 mm.; 

 dorso-ventrally, 25 mm. ; from side to side, 15 mm. (estimated). 



Locality. — Lower Llandovery of Mulloch Hill. 



Observations. — A single specimen, a cast of the left valve, is in the collection. The 

 hinge plate is most perfectly preserved, and all details of the interior are exposed. The 

 inferior part shows some details, however, of the surface marking. 



Cyrtodonta Billing siana, Salter, 1866. (PI. IV., fig. 18.) 



Palxarca Billingsiana, Salter, 1866, p. 342, woodcut 12, fig. 4. 



Specific Cliaracters. — Shell rhomboid, ovate, moderately gibbose, inequilateral. The 

 anterior end the shorter, compressed at its antero-superior angle, which is a right angle. 

 The anterior margin is sinuous above, bluntly rounded below. The inferior border 

 broadly convex. The posterior obliquely truncate, nearly straight ; the postero- 

 superior angle well defined, somewhat obtuse. The hinge line is almost straight. The 



