Hyatt*] 
336 
Dec. 2. 
of making the shells useful in geologic and paleontologic research. 
The want of sufficient materials of the younger stages of modern 
forms and shells of all ages in the Carboniferous is evident, and I 
hope that my results will prove of such interest to those having 
collections, that they will be disposed to aid me in further 
researches. 
PINISTIDAE. 
All of this group 1 whose interiors have been seen have the an- 
terior muscle divided into two parts, either by a distinct ridge or 
by a deflection of the lines of growth in the nacre. Sometimes 
the median line is very slight but it can generally be seen 
especially on the posterior border of the impression. The divid- 
ing line appears to have some connection with the carina until it 
is found to be present in Atrina as well as in other genera with- 
out carinae. 
The nacreous layer is thicker proportionately in the young as 
compared with the fibrous layer, the latter being excessively thin 
in the young of some shells. The nacre thins out posteriorly 
and is replaced finally by the fibrous layer. The fibrous layer 
alone often occupies more than one half the entire length of the 
valve. 
The length of the nacre on the dorsal area is apparently gov- 
erned largely by the position of the posterior muscle, or perhaps 
it is merely a correlative character dependent upon the same cause, 
since the two do not invariably coincide so closely as they do in 
A. nigra and some other common forms. There are certainly 
two species in Atrina, ex. A. seminuda , identified as figured by 
Reeve, and an unnamed form allied to a smooth variety of A. 
pectinata , and others in Cyrtopinna mentioned below in the 
description of that genus, in which the nacreous layer of the 
dorsal area extends considerably beyond the posterior borders of 
the posterior muscular impressions. In most species of Pinna also 
there is a slight but distinct extension of the nacre of the dorsal 
area beyond the posterior limit of the muscle. The length of the 
nacre on the ventral area is variable, being sometimes longer than 
1 The muscular impressions of Aviculopinna are so slight that they cannot be seen 
on the casts. Those of Sulcatopinna are also very slight but they do not differ, mate- 
rially at least so far as the posterior ones are concerned, from those of Pinna. 
