NAIADITES MODIOLARIS. 133 



diverging grooves are seen on the posterior slope, which apparently rise from the 

 upper part of the keel, and are seen as ridges in casts. The hinge-plate is striated 

 (in some specimens with as many as ten striae) in the anterior two-thirds ; it then 

 becomes narrowed as it passes backwards, and finally at about half the length of 

 the hinge-line disappears, and the flattened and thinned upper border of the 

 opposite valve comes in contact. The lowermost striae are the most prolonged, the 

 hinge-plate being thickest at the anterior end. In front above the stria? of the 

 hinge-plate is seen the inner surface of the umbo, which overlaps slightly inwards 

 in young and unworn specimens. These striae appear to represent the bevelled 

 edges of the plates of which the shell is composed, having been deposited in succes- 

 sion from the mantle, and the innermost being the largest. They can be traced 

 downwards for a short extent into the anterior border, but soon become lost, 

 and have entirely disappeared at the byssal notch. 



Exterior. — The surface is ornamented with stria? and lines of growth ; the 

 striae may almost be said to be obsoletely imbricated. These lines of growth 

 arise at the apex of the shell, and are densely crowded. They pass downwards 

 parallel to the edge of the shell with a slight divergence until they reach the 

 oblique ridge, when they become separated by wider and regular intervals, and, 

 reflected rapidly at a right angle, they pass across the posterior part of the shell 

 to the superior border. Periostracum thick and wrinkled. Shell thick relatively 

 to its size. Its greatest thickness is 



Dimensions.—?!. XVII, fig. 17 : 



Greatest antero-posterior . . .32 mm. 



Greatest dorso-ventral . • .21 mm. 



Greatest thickness . . • .14 mm. 



Localities. — England : — The roof of the Hard-mine, Banbury, Holly Lane, and 

 Moss on Easling Coals, and Knowles or Winghar and Burnwood Ironstones of 

 North Staffordshire. White flats ; Coalbrookdale. Thirty feet below the Arley 

 Mine, Rochdale. Stubb's Mine, Ashton-under-Lyme. Above the Shale Coal and 

 Stanley Main, Wakefield. Above the Brockwell Seam, Wigglesworth. Codnor 

 Park, Derbyshire. South Wales :— Darren Pins. Scotland : — Annandale ; 

 Kilwinning; Shotts; Shettleston ; Splint Coal, near Glasgow ; Clyde Pits. 



Observations.— This species was first described under the generic name of 

 Avicula by Sowerby {op. supra cit.). Captain Brown, in the ' Fossil Conchology,' 

 copied Sowerby's descriptions, adding only a few words, but gave entirely different 

 figures. He then figured and described under the names Modiola funata and 

 M. mbtruncata forms which are so much like the original drawings of Sowerby 

 that I have given them as synonyms. This generic reference was, however, much 

 more correct than the original one of Avicula. 



I have very little doubt that the Mytilus Wesemselianus of de Ryckholt from 



