152 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
Dimensions : 
Antero- Dorso-ventrally From side 
posteriorly. (greatest length). to side. 
Pl. XIX, fig. 20, measures 50 mm. 35 mm. 30 mm. 
Pie ies Tle . 45 mm. 30 mm. 23 mm. 
Locality.—Scotland :—Black Band Ironstone, Bankend, Lesmahagow. 
Observations. —Lam indebted to Dr J.8. Hunter, of Braidwood, for the loan of 
and permission to describe and figure these specimens, which are second in size only 
to N. crassa of the Carboniferous Limestone. These shells can be distinguished 
from all others by the alternate fluting of the valves at the posterior inferior 
angle, a character which is not very apparent in young specimens (Pl. XIX, 
figs. 17 and 19), but becomes more developed with age. 
The shape and general appearance of the shell at first suggested a reference to 
Mr. Etheridge’s Anthracoptera obesa, but from this form N. magna is distin- 
cuished by its more obtuse gibbosity, not so nearly central, and the folding of the 
posterior inferior angle of the shell, a character | have observed in no other 
species of this genus. 
Mr. Etheridge says that the posterior margin of his shell is sigmoidal, and this 
is shown only in one of his figures ; but this curve refers to the margin only, and 
must not be thought to apply to the peculiar crumpling of the posterior inferior 
angle of N. magna, a character which is shown to be absent in all figures of 
N. obesa. 
N. magna occurs at a much higher horizon than N. obesa, having been obtained 
up to the present time only from the Blackband ironstone, placed, in the 
‘ Geological Survey Memoirs,’ Explanation of Sheet 23, p. 80, at the base of the 
middle beds of the Carboniferous Limestone Series. Judging from the Bankend 
section given at p. 32 of the same memoir, there is only one Blackband ironstone, 
the position of which is given about the centre of these middle beds; and at 
p. 26 the main Lesmahagow gas coal, which is stated at p. 80 to overlie the 
Blackband ironstone, is again given as follows : 
Ft. Inch. 
Upper Group ‘ , : : 2 2ods nos 
Strata with their coals and clay ironstone 7 L635) 0: 
Middle Group 4 Main Lesmahagow Gas Coal : : Teo: 
Strata. : : 2 Se loses 00); 
Lower Group 5 : : - a eos 
