Vol. 5I-] FRESHWATER LAMELLIBRANCHIATA. 347 



Horizons. — Freshwater deposits of the Devonian, Carboniferous, 

 and Permian systems. 



In studying the Carboniferous and Permian Anthracosidae in 

 1892, I found it impossible to retain the Palaeozoic forms described 

 by Forbes, Ludwig, and Geinitz as Anodonta, but included them 

 under the generic name of Naiadites, Dawson. My reason for 

 adopting this was that Dawson, under the term Naiadites, described 

 and figured six forms (' Acadian Geology,' 2nd & 3rd eds.), 1 which 

 he named : — N. (Anthracoptera) carbonaria, N. (Anthracomya) 

 elongata, N. (Anthracoptera) la&vis, N. arenacea, N. ovalis, N. angu- 

 lata. Therefore we see that Dawson's Naiadites includes three 

 groups of organisms : Modiola-like Anthracomya, Salter (1861), and 

 Anthracoptera, Salter (1863), and Anodonta-Wke properly Naiadites 

 (N. arenacea, N. ovalis, N. angulata). Salter having already 

 established the generic names Anthracoptera and Anthracomya for 

 the first two groups, I substituted Naiadites only for tbe last — that is, 

 for the Anodonta-like shells. But Dawson, according to Dr. Wheelton 

 Hind, 2 and in a more recent paper, 3 considers it best to drop out 

 Salter's genus Anthracoptera (1863) and replace it by Naiadites. The 

 third group, the Anodonta-like shells (N. arenacea, N. ovalis, N. an- 

 gidata), which at first he held to be true Naiadites, he considers now 

 as follows : — the two species N. arenacea and N. ovalis he refers to 

 Anthracomya, giving them different diagnoses and drawings i from 

 those that he had previously given, and one species (N. angulata) 

 he refers to Anthracosia. In consequence of this the Anodonta- 

 like forms, which I described in 1892 as ' Naiadites, Dawson,' 

 remain now without a generic name, so that I venture to establish for 

 them the new genus Palceanodonta. This differs from Anthracomya, 

 Salter, (1) by having equivalve shells always closed at both ex- 

 tremities ; (2) by having a narrow posterior end (in typical forms) ; 

 (3) by its curved, angulated (seldom straight) hinge-line ; (4) by 

 the disposition and shape of the muscular scars being quite 

 similar to those in Carbonicola, M'Coy, Anthracosia, King, or 

 Palceomutela, mihi. 



Pal^ianodonta okestsis, mihi. (PI. XIII. fig. 6.) 



1892. Naiadites okensis, Amalitsky, Palaeontographica, vol. xxxix. p. 193, 

 pi. xxii. figs. 26-29. 



This shell agrees completely with the diagnosis and the original 

 specimens themselves of this species, which are in my possession, 

 from the Permian marls and sandstones at Nishny-Novgorod, 

 horizon C. 



Dimensions : length 17, breadth 18 ; Russian forms : length 20, 

 breadth 10-11. 



Locality. South Africa : Bloemkop, near Graaf Peinet. 



1 See PI. XIII. figs. 7-10, 12, 14. 



2 Quart. Journ. Geol. Soc. vol. 1. (1894) p. 437. 



3 Canadian Record of Science, Oct. 1894, ' Rev. of the Bivalve Mollusca of 

 the Coal-Formation of Nova Scotia.' 



4 See PI. XIII. figs. 11 and 13. 



