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. 
Interiovr.—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, Pl. XVIII, gives the following details : 
Dorso-ventral : : F . 32mm. 
Length of hinge-line . : ; . 22 mm. 
Thickness . j : : . 13mm. 
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- 
