54 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



originals of Sowerby's types, which are in the " Sowerby Collection " in the 

 British Museum (Nat. Hist, branch). Fig. 1, PL III, is to be regarded as the most 

 typical ; but I question whether figs. 2 and 3, PI. Ill, do not really belong to 

 C. subconstricta, being rather elongated forms of that species ; because the 

 umbones in these two specimens do not rise gradually from the shell, as in PI. Ill, 

 fig. 1 (and this I have regarded as one of the specific features in G. acuta), but are 

 sharply marked off from the surface, as in the specimen of G. subconstricta, PI. VII, 

 figs. 5 — 15. Indeed, on comparing Sowerby's second specimen of his Unio acutus 

 (PI. Ill, fig. 2) with his specimens of U. subconstrictus (PL VII, figs. 5 and 6), little 

 if any distinction can be made between them. M'Coy (' British Palseoz. Fossils,' 

 p. 515) considered that the Unio aquilinvs of Sowerby (' Geol. Coalbrookdale,' 

 pi. xxxix, fig. 12) was identical with G. acuta; but he was in error in this idea, 

 not recognising the characteristically different shape and proportions of the anterior 

 end and the general obliquity of the shell of G. aquilina ; indeed, there should be 

 no difficulty at all in distinguishing between these two forms. 



Few workers in this field have considered it possible that young and immature 

 forms might be met with, and been able to resist the temptation of inventing fresh 

 specific names for such ; and amongst these, Brown, Ludwig, and Achepol are 

 conspicuously to the front. I think, from a personal observation of the original 

 specimens, there can be no doubt that Ludwig's Unio Lottneri is an elongate 

 variety of the species under description. 



Salter's figures in his ' Iron Ores, South Wales, &c.,' pt. 3, pi. ii, figs. 20, 21, 

 do not possess the straight margin of the posterior part of the ventral border. 

 In these two figures the anterior and posterior ends approached more closely to 

 Martin's Mya ovalis and Sowerby's Unio centralis; but the shells are much elongated 

 transversely. Unio Lottneri, Ludwig, is probably identical with Salter's figures. 



The great variation in the form of the hinge is very difficult to understand. 

 The hinge-plates figured on Pis. V and VI, with the exception of PL V, 

 figs. 1, 2, 32, and PL VI, figs. 43 and 44, are all from the same bed — a band of 

 ironstone above the Cockshead Coal, and were obtained by myself from an old pit- 

 mound at the Hulme Colliery, where they had weathered out of the matrix in a 

 fragmentary condition ; the hinge itself, owing to its thickness, generally being 

 preserved. I have been able to obtain a very large number of examples, very few 

 of which are exactly alike. Besides the three forms I have ventured to describe, 

 and those showing intermediate features, there are others, of which figs. 4, 5, 6, 

 12, and 13, PL VI, may be taken as examples, showing a total departure from the 

 other forms, and the absence of anything like a cardinal tooth. PL VI, fig. 6, 

 shows the anterior two-thirds of the hinge of the right valve ; and all the articu- 

 lating arrangement present is a bevelling of the lower edge of the hinge-plate. 

 In other cases the hinge-plate has been much hollowed out, PL VI, figs. 13 — 16. 



