NAIADITES OBESA. 145 



Interior. — Anteriorly there are only two muscle-pits, but the lower is very large, 

 and probably represents the two lower scars of other species. The upper or 

 smaller scar is in its proper situation. The posterior adductor muscle scar has 

 not been noted. The pallial line is represented by a series of small pits. 



The hinge-plate is striated longitudinally, broad in front, becoming gradually 

 narrow behind. In front the lower edge is thickened and projects as a cardinal 

 tooth, that of the left valve being anterior and above. The anterior edge of the 

 shell is thickened and bevelled at the expense of its outer surface, exposing the 

 layers of shell-matter. 



Exterior. — The surface is ornamented with lines of growth, which, crowded in 

 front, soon separate and become parallel to the margins. 



Dimensions. — PI. XIX, fig. 1G, measures: 



Antero-posteriorly . . . .34 mm. 



Dorso-ventrally . . . .25 mm. 



Localities. — Scotland : — Micaceous sandstone, Drumsheugh. Water-of-Leith 

 and Dean Bridge, Edinburgh, in the Wardie Shale group of the Cement-stone series. 

 Corstorphine Hill (shale), in limestone above the sandstone of Craigleith quarry, 

 Edinburgh. Shale above the " Brownstone " portion of the Boness lower iron- 

 stone Edge-coal series, Duncan pit. Shingle pit and Cowsiemine, Boness. Zone 6, 

 40 feet below the Encrinite bed, east of Pittenweem. Zone 11, east of Target, 

 Billowness. Zone 13, below sandstone at Billowness. Zone 14 and 15, east of 

 Billowness. Anstruther Wester, zone 17. Zone 18, west of Dreel Burn. In 

 ferruginous sandstone, opposite Kilrenny mill. Limestone Nos. 2 and 3, Ran- 

 derstone, east of Kingsbarns' Harbour. Above the bed of coal at the Pock and 

 Spindle, uear St. Andrews, all in the Calciferous sandstone series of the Fifeshire 

 coast. 



Observations. — This form is sufficiently different from others to merit a specific 

 name, and it appears to be confined to the Lower Carboniferous beds, occurring in 

 the Calciferous-sandstone series and Middle Limestone series of Scotland, and is 

 therefore the earliest known form of this genus in Carboniferous beds. Naiadites 

 obesa was evidently gregarious. There are beds on the Fifeshire coast, of one or 

 two feet in thickness, composed of masses of this shell, which extend for a 

 considerable distance. After much laborious field-work, Mr. James W. Kirkby has 

 been able to gather, from portions of such beds which have become decomposed 

 by the action of air and sea-water, several fragments from which the internal 

 anatomy of the shell has been made out, so that the characters of this, the 

 earliest known form, are known even better than the more recent. 



Mr. Kirkby at first gave the MS. name of Myalina communis to this shell, but 

 subsequently, in his paper (op. supra cit.) he doubtfully referred it to Modiola 

 modioli/oralis, Brown, ' Foss. Conch.,' pi. lxvi, fig. 19. This, however, Mr. R. 



19 



