98 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
Casts of A. modiolaris show no indication of teeth of any description, and have 
exactly the same characters as casts of the other species of this genus. It is, 
therefore, not safe to come to any conclusion as to the absence of hinge-teeth 
in other forms from an examination of the casts alone. 
This species is easily recognised by its narrow-elongate gibbose form. It is 
much narrower in a dorso-ventral direction than any of the foregoing species, and 
can be distinguished from A. Williamsoni by its greater gibbosity and more 
pronounced oblique ridge; it generally, too, attains a greater size. These two 
shells do not, according to my experience, occur together. In North Staffordshire 
each species is limited to its own bed, A. modiolaris occurring in the roof of the 
Holly Lane Coal, an horizon some little way above the Hard Mine, where 
A, Williamsoni occurs. In Scotland the species occurs in very typical forms in 
the Upper Coal-measures of Lanark and Ayrshire. Mr. Etheridge, jun., stated 
that it was the only member of the family recognised in Scotch beds, and, indeed, 
it seems to be the most common. (Presidential Address, Royal Physical Soe. of 
Scotland, the ‘ Paleozoic Conchology of Scotland,’ 1882, p. 50.) He says, ‘ It 
is confined to the Coal-measures, and the Middle Coal and Ironstone Series.”” In 
these beds A. modiolaris always occurs in the form of casts. The figure of Capt. 
Brown’s Modiola curtata is very similar to the specimens figured on Pl. XIII, 
fig. 10. There are some specimens of the same form in the Owens College 
Museum, locality not stated, but probably from Lancashire beds, as I know the 
form from the horizon of the Black Mine, Oldham. Captain Brown’s specimen 
came from Wakefield. His figure is, I think, one of a somewhat truncate form 
of A. modiolaris, but owing to the fact that I have been unable to see many 
specimens I have not considered it wise to give a varietal name to these shells. 
Should it be shown in the future that the form exists as a well-marked local 
or horizontal variation, Capt. Brown’s name might be given as a varietal name, 
A. modiolaris, var. curtata. 
It is rather more doubtful whether (judging from the figure alone) M. Robert- 
soni, Brown, should be considered as a synonym of A. modiolaris, but the 
description, when correct according to the figure, agrees very well. ‘‘ Elongated, 
inflated ; beaks subcentral (which they certainly are not in the drawing), obtuse, 
rounded. Hinge-line short (not so in the drawing). Surface smooth, with 
remote shallow lines of growth.” If the original was intended for a member of 
this genus, I know no species to which it bears more resemblance than the one 
under discussion, and A. modiolaris occurs in the Neweastle-on-Tyne Coal-field, the 
locality from which Captain Brown obtained his shell. 
I figure on Pl. XIII, fig. 14, a large shell from the Collection of Mr. J. Smith, 
of Kilwinning, obtained from Old Cummock, which is probably a giant form of 
A, modiolaris. He has lent me a slightly larger one, which measures 60 mm. 
