NAIADITES CRASSA. 149 
slightly bevelled at the expense of its upper edge; longitudinally striated, in 
old specimens as many as fifteen striz; these, thickened and less defined, are 
continued round to the thickened, anterior border. In front the lower edge of the 
hinge-plate is thickened forming an ill-defined tooth, that of the left valve being 
anterior to that tooth in the right. 
The shell substance is very thick, and in front encroaches much on the cavity 
for the animal. Posteriorly it becomes much thinner, and is rarely preserved. 
The apex of the cavity of the shell does not at all correspond with the exterior, 
being below and behind the real anterior point of the shell. 
Surface ornamented in a similar way to that of other members of the genus. 
Dimensions.—P|. XX, fig. 10, a cast of the interior, from Beith, measures : 
Extreme antero-posterior diameter ; ; (6 min: 
Greatest dorso-ventral (posterior end) é . 48 mm. 
From side to side ; s . 30 mm. 
Localities.—Scotland :—Cult’s Limeworks near Pitlessie, Fife, in bed of shale 
over the Mountain-Limestone. Lugton Water, near Dunlop, Ayrshire ; Lower 
Carboniferous Limestone, Woodhall; Water of Leith. Roughwood and Lyon- 
shields, Beith, in a shale below the Main Post of the Lower Limestone Series. 
Observations.—I have already referred to the reasons why I have placed this 
shell in the genus Naiadites, and have discussed the question of its habitat. 
Naiadites crassa has been known to paleontologists since 1825, but has only been 
figured by Dr. Rhindand Mr. R. Etheridge, jun. I fancy that the former has figured 
the smaller or dwarfed form (from the Water-of-Leith), noticed at p. 150. Mr. 
Etheridge’s figures and description are very good and typical. In his observations, 
however, he quotes M‘Coy’s opinion as to the third or uppermost of the anterior 
adductor scars being for the insertion of the adductor of the opposite valve. He 
states too: ‘‘ immediately in front and within the angle formed by the hinge-plate 
and the anterior margin is another shallower depression, from which a depressed 
and more or less interrupted line runs in many specimens across the cartilage area, 
sometimes even interrupting the furrows themselves.” He suggests that the rim or 
margin, described above, represents the rostral plate of Myalina and Dreissenia. 
This line and depression are very often absent, and I am inclined to attribute the 
presence of them to accidents of growth and environment. 
The only member of the genus Naiadites which attains anything like the size 
of N. crassa is N. magna, but they are very dissimilar, and occur at very different 
horizons, though both forms have as yet been found only in Scotland. 
It is curious to note how very many specimens of this shell obtained from the 
Cults beds have been damaged and repaired during life. ‘his mussel-bed must 
have occupied a very exposed position, perhaps off some headland, where they 
were at the mercy of a strong tidal wave; and in this bed it is very rare to find 
