214 F. B. Meek on the Family Pteriide. 
seem to show a disposition to shade off towards the Mytilide or 
Dreissenide. Amongst the Carboniferous and Permian species — 
of Myalina, for instance, we see species presenting exactly the — 
form and other general external appearances of the existing 
genera Mytilus and Dreissena, to which even yet some paleon 
tologists will persist in referring them. On a closer inspection, 
‘however, these Carboniferous and Permian species, when we call 
find them with the two valves united, are seen to be always 4 
little inequivalve, while their hinge also differs from that of the 
Mytilide and Dreissenide in having a flat area with longitudinal 
cartilage furrows. In addition to these differences, 1 have dis 
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in the true Aviculas. It is true that the same structure has also 
been observed by Dr. Carpenter in the inner layer of Dreissend, 
but the unquestionable imequivalve character of Myalina, 10 coh 
nection with its peculiar striated cartilage area, and the fact 
these shells are always found associated with marine types, 7? 
sufficient evidences that they have no very close affinities © 
Family PTERIID.A (or Aviculide). 
Shell inequivalve, inequilateral, composed of an inner. lam 
nated pearly layer, and an outer prismatic substance; left ot 
upper valve always more convex than the other. AW 
margin of the right valve generally more or less sinuous er 3 
passage of the byssus. Cartilage submarginal, simple, and Ee ee 
‘In a single cavity or depression near the beaks, or divided s : 
distributed in a series of furrows crossing the cardinal fact 
right angles,—or, in some of the older fossil genera, occupy 
linear furrows in the cardinal area or facet, ranging more or wed ; 
— parallel to the hinge line. Hinge with or without pers 
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muscular impression generally small and placed near the D sila 
Pallial line simple, often irregularly dotted. ae 
Since Scopoli’s namie, Pteria, was regularly established in 17795 While iy 
pame, Avicula, was not affirmed by any om Ae understanding oF 789. I b 
= n ideas of genera and species, until Bruguiere adopted it 10 1789. 
- Gray is right in restoring Scopoli’s name, Pteria, for this genus. 
