30 PALAEONTOLOGY OF THE UPPER MISSOURI. 



several of these ancient types is provided with cartilage furrows, as in the Arcidce. 

 Again in Pterinia, and some species of Bakevellia, we see the anterior muscular 

 impression comparatively so well developed, that one can scarcely believe it was 

 not made by a true adductor ; while the eccentric position of the posterior muscular 

 impression would seem also to favor the same conclusion ; and yet in all their other 

 known characters these forms agree with the Ariculidce. 



In another direction, some of these ancient groups of Aviculidai seem to show a 

 disposition to shade off towards the MytilidoK or Dreissenidce. Amongst the Carbo- 

 niferous and Permian species of Myalina, for instance, we see shells presenting 

 apparently exactly the form and general external appearances of the existing genera 

 Mytilus and Dreissena, to which even yet some Palaeontologists will persist in re- 

 ferring them. On a closer inspection, however, these Carboniferous and Permian 

 species, when we can find them with the two valves united, are seen to be always 

 a little inequivalve, while their hinge also diifers from that of the Mi/tilidce and 

 Dreissenidce, in having a flat cardinal area, with longitudinal cartilage furrows. In 

 addition to these differences, we have ascertained that the shell structure of at least 

 two species of Myalina (M. perattenuata, M. & H., and M. angnlata, Meek & 

 Worthen) is minutely prismatic, as in true Avicida. 1 It is true the same struc- 

 ture has also been observed by Dr. Carpenter in the inner layer of Dreissena ; 

 but the unquestionable inequivalve character of Myalina, in connection with its 

 peculiar cardinal area, and the fact that these shells are always found associated 

 with marine types, are sufficient evidences that they have no very close affinities 

 to Dreissena. 



SUBFAMILY PTERINIIN^E. 

 Genus MYALINA, KONINCK. 



Synon. Mytilus (sp.), SOWEKBY, and others (not LAMAECK, 1801). 



Myalina, KONINCK, An. Foss. 1842, 125 (not CONKAD, 1845). 



Aucella, GEINITZ, Dyas, 1861 (not KEYSEKLING, 1846, nor McCoy, 1855). 

 Etym. ? Mya. 

 Examp. Myalina lamellosa, KONINCK. 



Shell mytiliform, or subrhomboidal, extremely inequilateral, moderately inequi- 

 valve, more or less oblique ; valves apparently a little gaping and slightly sinuous 

 in front for the passage of the byssus. Beaks pointed and nearly or quite terminal; 

 sometimes provided with an internal shelf or septum apparently for the attachment 

 of the anterior muscle. Surface smooth or with concentric markings of growth, 

 which, in some species, form imbricating laminae. Hinge nearly or quite edentu- 

 lous ; ligament area usually broad, and marked by distinct cartilage 1 furrows 

 parallel to the hinge line. Muscular and pallial impressions apparently as in Pteria. 



Animal unknown. 



1 See Am. Jour. Sci. XXXVII, March, 1864, 214. 



