68 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



good figures (op. supra cit.), but in this same paper described, under the 

 names of Pachyodon dubms and P. similis, two shells which I consider to belong 

 to the same species. His figure of P. dubi/us resembles the specimen I have figured 

 on PI. 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 G. turgida, 

 PI. 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 is 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 striae 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 G. 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. 15 a, PI. 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. 



