68 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
good figures (op. supra cit.), but in this same paper described, under the 
names of Pachyodon dubius and P. similis, two shells which I consider to belong 
to the same species. His figure of P. dubius resembles the specimen I have figured 
on Pl. VIII, fig. 17, from the collection of Dr. John Young, and which I regard 
as a young example of this species. 
The description given by M‘Coy is very accurate and striking. He states 
that “this species is easily distinguished from all others by its short quadrate 
form and evenly gibbous, very tumid valves.” This form does not appear to have 
been recognised by Mr. Salter; for he figures a specimen of it (op. supra cit.), 
under the name of Anthracosia ovalis (Martin), which shows not the slightest resem- 
blance to Martin’s original figure. 
Mr. Ward figures a somewhat imperfect specimen from the roof of the Moss 
Coal (op. supra cit.) under the name Anthracosia centralis, but it has the tumid 
prominent umbones of the species under notice. I have figured a number of 
crushed and smaller forms from this bed, which may be young of C. turgida, 
Pl. VIII, figs. 21—25. The peculiar shapes in these cases are due to crushing-in 
of the lower edges of the shell and separation of the valves, but they all possess 
the characteristically shaped posterior slope and prominent tumid umbones. This 
species 1s not at all common, and where it occurs has a very small vertical range ; 
but it seems to have had a wide horizontal distribution. It is one of the most 
elegant of the Coal-measure shells when perfect ; but it is often difficult to recog- 
nise when crushed. The form, too, is fairly constant; but it will be observed that 
the obliquity of the striz so marked in some specimens (PI. VIII, figs. 8, 12, and 
18) is not always present ; figs. 10, 13, 19, and 20 showing an almost perfect con- 
centric arrangement. Whether this variation is due to locality I cannot say, but 
all the specimens which have come into my hands from Coalbrookdale and most 
of those from North Staffordshire are not obliquely grown. Most of the Scotch 
forms appear as casts. 
The form is closely allied to C. gibbosa, which perhaps may be only a giant 
variety ; but there are certain differences, which have induced me to separate the 
two forms. 
The cast of the hinge-line, fig. 15a, Pl. VIII, indicates by the S-shaped flexure 
between the umbones the presence of cardinal teeth. 
The name turgida has been adopted because the shells figured by Captain 
Brown under this term are more typical than those figured under those names 
which I have regarded as synonymous. 
