NAIAD1TES CRASSA. 149 



slightly bevelled at the expense of its upper edge; longitudinally striated, in 

 old specimens as many as fifteen strife ; these, thickened and less denned, 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. — PI. XX, fig. 10, a cast of the interior, from Beith, measures : 

 Extreme antero-posterior diameter . . 70 mm. 



Greatest dorso-ventral (posterior end) . . 48 mm. 



From side to side . . . .30 mm. 



Localities. — Scotland : — Cult's Lime works 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. 

 Naiaclites crassa has been known to palasontologists since 1825, but has only been 

 figured by Dr. Rhind and 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. This 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 



