82 Dr. E. von Martens on the Subdivisions of Pinna. 
which a part of the fibrous layer, the only one preserved, 
formed a very striking tongue-like process, pointing towards 
the apices, and easily to be traced by the lines of growth. He 
inquired if anything like it was known in the recent species ; 
and on examination of those contained in the Berlin Zoological 
Museum, we found that in a considerable number of them the 
nacreous layer is divided in the middle line of each valve by 
a much narrower, but very distinct, line exhibiting the same ap- 
pearance as the fibrous layer of the posterior part of the inside. 
This line is often prolonged very nearly to the apices, but never 
reaches them; its end gives clear evidence of becoming gra- 
dually covered with the same nacreous layer which on both 
sides of it extends to a much greater distance from the apices. 
As the described line forms a sinus incurving into the space oc- 
cupied by the nacreous portion, and beginning just at the side 
of the large muscular impression, and as the border of the na- 
creous part itself seems to be the line of slight adherence of 
the mantle to the shell, corresponding to the pallial line of other 
bivalves, the above-mentioned incurrent line may be compared 
to the pallial sinus of the Veneride (especially that of Artemis) 
and other familes, and may hence be called simply a sinus. 
In some specimens of Pinna preserved in spirits there is to be 
found a corresponding line in the mantle itself; but I am not 
prepared to state whether it may be a muscular organ, nor do I 
find any satisfactory explanation of it in Poli’s detailed anatomy 
of Pinna, where, however, the sinus of the inside of the shell is 
very well figured. u 
In most specimens provided with this interior sinus, there is 
to be seen outside near the apices the longitudinal fissure sisted 
upon by the brothers Adams ; nor did I tind any specimen pro- 
vided with this outside fissure wanting the interior sinus. The 
fissure seems to begin at the apices, and becomes more and 
more obsolete as its distance increases, 7. e. during the further 
growth of the shell. Its situation is outside, just opposite to the 
internal sinus, or to the prolongation of it towards the apices. 
The matter which fills up the outside fissure has quite the ap- 
pearance of the fibrous portion of the shell; and I satisfied my- 
self, by the application of a dilute acid, that it contains car- 
bonate of lime, as also the other constituents of the shell ; there- 
fore it should not be named cartilage. 
There can be no doubt, I think, about the causal connexion 
between the imside sinus and the outside fissure, or, rather, a 
common cause of both, from which results that for some time 
no nacreous matter is deposited in the middle line of each valve. 
Taking into consideration all these circumstances, it seems not 
improbable that the external fissure is nothing else than the 
