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 striz) 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 striz are the most prolonged, the 
hinge-plate being thickest at the anterior end. In front above the striz of the 
hinge-plate is seen the inner surface of the umbo, which overlaps slightly inwards 
in young and unworn specimens. These striz 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. 
Hexterior.—The surface is ornamented with striz and lines of growth; the 
strize 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.—P1. XVII, fig. 17: 
Greatest antero-posterior : ; 5 Se) mayan, 
Greatest dorso-ventral : ‘ 5 PAL saaian, 
Greatest thickness : 14 mm. 
Localities. —England :—The roof of the Hard-mine, Banbury, Holly Lane, and 
Moss on Kasling 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. subtruncata forms which are so much like the original drawings of Sowerby 
that I have given them assynonyms. This generic reference was, however, much 
more correct than the original one of Avicula. 
I have very little doubt that the Mytilus Wesemxlianus of de Ryckholt from 
