508 DR WHEELTON HIND ON 



conditions of growth. The peculiar external character is well marked in fig. 30, PI. II., 

 and one example, fig. 31, PI. II., gives all the details of the interior except the 

 hinge plate. 



Modiolodon quadratus, sp. nov. (PI. II., figs. 28, 29.) 



Specific Characters. — Shell below medium size, transversely quadrate, compressed, 

 anterior end almost obsolete, depressed. The inferior border is almost straight, parallel 

 with the upper border. The posterior border very bluntly rounded. The hinge line 

 depressed anteriorly, straight posteriorly. The umbones are anterior and immediately 

 above the anterior end, so that they point directly forwards and are apparently placed 

 half-way up the anterior margin, but which really is the junction of the small anterior 

 end and the depressed anterior part of the hinge plate. 



Interior. — The anterior adductor scar is large and is placed low down at the antero- 

 inferior angle just within the margin. Hinge plate not seen. 



Exterior. — The surface is almost smooth but concentric, fairly regular, but distant 

 plicae are seen towards the lower margin and over the posterior part of the valve. 



Dimensions. — Fig. 28, PI. II., a left valve, measures : antero-posteriorly, 10 mm. ; 

 dorso-ventrally, 15 mm. 



Locality. — Woodland Point, Middle Llandovery. 



Observations. — A very quaintly shaped shell which should not be mistaken. It is 

 allied to the M. obtusus, Ulrich, but the latter is much more transverse and not so 

 broad in front. The Megalomus, Cypricardites Deshayesiana of de Verneuil, is much 

 too gibbose and has a well-marked umbonal ridge. Three specimens are in the collec- 

 tion, one of which, a left valve, shows the anterior adductor muscle scar. 



Modiolodon subcircularis, sp. nov. (PI. II., figs. 26, 27.) 



Specific Characters. — Shell below medium size, subcircular, slightly oblique, com- 

 pressed, inequilateral. The anterior end has its margin rounded and continued in an 

 interrupted subcircular sweep to the postero-superior angle. Hinge line arcuate. 

 Umbone small, compressed, pointed, placed a little in front of the centre of the valve, 

 raised. The valve is moderately convex, with a gently compressed dorsal slope. 



Interior. — The adductor muscle scars are large, deep, and normal in position. 

 Pallial lines entire. Hinge plate has no lateral teeth. Cardinal teeth not exposed. 



Exterior. — The surface is covered with very fine concentric lines of growth, which, 

 towards the lower margin, become strengthened into concentric ridges and grooves. 



Dimensions. — Fig. 27, PI. II., a left valve, measures: antero-posteriorly, 21 mm.; 

 dorso-ventrally, 20 mm. 



Localities. — Middle Llandovery of Woodland Point and Newlands. 



Observations. — This species, which shows strongly marked generic affinities with 

 M. subovalis, with which species it also occurs, differs so markedly in general contour and 



