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[Hyatt 
1 S 91 .J 
vonian than other species of the same genus. Pteronites leads 
into Leptodesma and the latter, as pointed out by Jackson, has 
affinities with both the normal forms of Pinnidae and Aviculidae. 
The series seems therefore to be Leptodesma, Pteronites, and 
Aviculopinna, forming a phylum which ends in Aviculopinna* 
The two last named genera have the umbones nearly terminal 
and until the young of Atrina and Pinna are better known, they 
had perhaps better be considered as included in the family of the 
Pinnidae. 
As pointed out by Dr. Jackson 1 the affinities of the young of an 
unknown species of the Pinnidae (I think probably a species of 
Atrina), as indicated by its extended hinge line and general out- 
line, show that Palaeopinna was probably the ancestral form or 
proximate radical. We differ in that I do not consider Aviculo- 
pinna as descended from Palaeopinna, but as coming directly 
from the more primitive Pteronites, and, therefore, a phylum 
distinct from that of the normal Pinnidae. In support of this 
opinion, it may be stated that the nepionic stage of the young of 
Aviculopinna must have resembled Pteronites in outline, since 
the lines of growth indicate this very closely in a specimen in the 
Museum of Comparative Zoology from the Carboniferous at 
Millersburg, Holmes Co., Ohio. 
All of the smooth so-called Pinnidae, of the Carboniferous are 
members of this genus. Besides those mentioned above, A. con 
similis sp. Walcott and probably Pinna ivaniskiana De Vern , 2 
A. spatliula McCoy and De Ivon, and A. d’orbigni De Ivon., 
can also be referred to this genus. 
Atrina was proposed by J. E. Gray , 3 the type being A. nigra 
sp. Chemn, equal to P. nigrina Lam., an unmistakable form, and 
it has not been clear to me why several conchologists have trans- 
ferred this generic name to the group of Streptopinna, taking 
S. saccata as the type. 
1 Phylogeny of the Pelecypoda, Mem. Bost. Soc. Nat. Hist., IY, p. 385. 
2 Described in Russia and the Ural Mts. as having carinae, but the figure and the 
lines of growth, if correctly given, show that the hinge line was mistaken for a lateral 
carina. 
3 Syn. Cont. Brit. Mus., 1840, name of genus was printed, but no type was mentioned, 
figure referred to, or description given. In List of Moll. Proc. Zool. Soc. Lond., 
1847 , p. 199, type P. nigra was cited, and also a work in 1844 referred to by date only, 
which I have not seen. 
