NAIADITES QUADRATA. 141 



anterior to the middle point, a slight sinuosity marking the aperture for the 

 byssus. The inferior border is narrow, being obliquely semicircular, and it passes 

 into the anterior and posterior borders in front and behind without any angle, 

 forming one general curvature. The posterior border is generally straight, and 

 meets the posterior extremity of the hinge-line at an angle varying from rather 

 less than 90° to 110°. The hinge-line is straight, as long as or slightly longer 

 than the greatest diameter of the shell parallel to it. The umbones are small, 

 inconspicuous, twisted forwards to be parallel to the hinge-line, not contiguous. 

 From the umbones there proceeds a strongly-keeled oblique gibbosity which is 

 directed downwards with a slight inclination backwards, and becomes expanded 

 and lost on the surface after it has traversed about half the depth of the shell ; 

 this is less pronounced in the right valve. The posterior end is flattened and 

 produced directly backwards. 



Interior. — Owing to the anterior end being so little developed, as in Naiadites 

 modiolaris, the three anterior muscle-scars are close together, and the posterior 

 adductor does not seem to be relatively so close to the hinge-line. Hinge- 

 plate thickened in front and striated. 



Surface-markings as in the three previous species. 



Dimensions. — Fig. 23, PI. XVIII, gives the following details : 



Dorso-ventral . . . .32 mm. 



Length of hinge-line . . . .22 mm. 



Thickness . . . . .13 mm. 



Localities. — Roof of the Hard-mine, Ten-foot, and Banbury seams; Little- 

 mine or Burnwood Ironstone Fenton ; above the Woodhead Coal, Froghall, in 

 North Staffordshire; Grains Ironstone, Oldbury, and Portway Hall, Dudley; 

 White-flats and Pennystone, Coalbrookdale. Netherton, near Morpeth ; Bunker's 

 Hill, west of Rochdale, thirty feet below the Arley mine. South Wales : The 

 Darrenpins. Scotland : Middle Coal-measures of Shotts. 



Observations. — This form was considered by Sowerby to belong to Avicula along 

 with Naiadites modiolaris ; why, it is somewhat difficult to conceive. His original 

 description was — "Quadrangular, convex, keeled; anterior lobe small, pointed ; 

 front rounded." The original specimen appears from the drawing to be incomplete 

 at its posterior superior angle; but it has the oblique ridge directed almost 

 downwards— what I regard as typical of this form. The word quadrangular is 

 somewhat misleading, as the figure well shows the oneness of the curve of the 

 inferior border with the anterior and posterior edges. The view in profile shows 

 a thicker shell than I have met with in North Staffordshire. 



Mr. Salter's figure from South Wales (op. supra cit.) is complete and very 

 typical. 



Captain Brown, in his ' Fossil Conchology,' copied Sowerby's figure and descrip- 



