CARBONICOLA ROBUSTA. 45 
erosions (see Pl. IV, figs. 8—12). It is curious, however, that I have been able to 
make out this peculiar feature in no other species of Carbonicola, but erosion 
is present in similar forms from other localities; and this eroded condition of 
the umbones is by no means universal, even in the bed mentioned. I conclude 
that erosion would not take place provided that the periostracum remained intact, 
and also that the presence of sufficient CaCO, to neutralise the free carbonic acid 
in the waters in which these shells lived would largely prevent erosion. It is 
still, however, a matter of great uncertainty as to what is the real cause of erosion 
in Unios; and, indeed, in some localities they are not thus affected. 
The nearly universal occurrence of truncate and somewhat ventricose varieties 
of each species suggests the question as to the possibility of the dicecious 
character of these Coal-measure Lamellibranchs; this, of necessity, is a question 
which cannot be settled in the absence of all data, though Ludwig and Achepol 
have ventured so far as to describe certain tumidities in the upper and posterior 
part of specimens as due to the situation of the ovary—a somewhat unwarrantable 
assumption, judging from what is known of the sexual organs of the Unionidz.' 
It is interesting to note that similar variation of forms occurs in the species of 
Anthracomya and Naiadites 
I have considered it unnecessary to raise Anthracosia to the dignity of a Family, 
as I think that of the Unionidz to be sufficiently wide to embrace a genus which 
seems to present only one important difference from the other genera included in 
it, viz. the abnormal (qud Unio) situation of the anterior-pedal muscle-scars. 
1. CarsonicoLa RopustTA (Sowerby). Plate I, figs. 1—6; Plate IT, figs. 1—6, 9—11. 
Unto rosustus, Sow. Trans. Geol. Soc., ser. 2, vol. v, pl. xxxix, fig. 14, 1840. 
Carpinta RoBustA, de Koninck. Anim. Foss. Belg., 1842-4, p. 71, pl. ii, fig. 1. 
Unio rogustus, Brown. Foss. Conch., 1849, p. 181, pl. lxxxviii, fig. 16. 
CaRBONICOLA SUBCONSTRICTA, var. RoBUSTA, ‘Coy. Brit. Paleozoic Foss., 1855, 
p. 515. 
Non Anruracosta Rronusta, Hull. Coal-fields of Gt. Britain, 1861, pl. i, fig. 1. 
— _— Salter. Mem. Geol. Survey, Country round Wigan, 
Sheet 89, p. 37, fig. 2, 1862. 
Non _ — Wardle. Geol. of Leek, 1863, pl. iv, fig. 18. 
ANTHRACOSIA CRASSA, Achepol. Niederrheinisch.- Westfilische Steinkohlen-Gebirge, 
Atlas, 1880—1883, Supp., pl. i, figs. 19, 20, 
1880-83. 
—_ caupata, Achepol. Ibid., Supp., pl. iii, fig. 49, 1880-83. 
— ropusta, Ward. Trans. North-Stafts. Inst. Min. Mech. Engineers, 
vol. x, p. 127, pl. i, fig. 1, 1890. 



1 Though generally said to have the sexes distinct, I have never by dissection been able to satisfy 
myself that such was the case in the Unionide. 
