Dr. E. von Martens on the Subdivisions of Pinna. 83 
inside sinus rendered visible by the destruction of the more 
external strata of the shell. I have never seen any of these 
fissured Pinne in which the apices were not worn off to a greater 
or less extent ; so that nothing can be stated about the apical 
beginning of the fissure. By the gradual destruction of the 
shell near the apices, externally, during life, and by the per- 
manent deposition of fresh nacreous matter inside, a stratum 
must become visible outside which has been formed inside, and 
must then show what has been the appearance of the inner face 
at some previous time, when this stratum was inside. This seems 
to be the explanation of the fact, striking at first, that the out- 
side fissure approaches much nearer to the apices than the inside 
sinus, and disappears much sooner in the opposite direction. In 
each individual at each stage of growth there must be observable, 
therefore, three regions :— 
1. A continuous region of fibrous matter, constituting (a) 
alone the abapical or later part of each valve, visible at the inner 
and outer face; (b) the outside or older strata of another part of 
the shell nearer to the apices, but entirely worn off still nearer 
to them. 
2. A region of nacreous matter on each side divided by the 
sinus of fibrous matter. This region is visible in the middle 
part of the imner face and in the apical part of the outer face of 
the shell, which is explained by the above consideration. 
3. A region of continuous nacreous deposition, visible in the 
apical part of the inner face, worn off generally on the outer face. 
All these regions expand with the advancement of growth on 
the imner face, the following covering the preceding. 
The fibrous matter of the sinus seems not only to be a part 
of the preexisting continuous fibrous layer not covered by 
nacre, but fibrous matter seems to be deposited here also con- 
temporarily with the nacre on both sides: this is to be concluded 
from the fibrous matter being on the same level with the nacre, 
as is clearly shown in the worn-off outside (the fibrous matter 
filling up the fissure), and fu. ner by the lines of growth dis- 
cernible in the fossil species with a broad sinus, these lines fol- 
lowing the outlines of the sinus. Hence it becomes probable 
that the above line in the mantle, corresponding to the sinus, is 
is not quite inactive, but secretes fibrous matter instead of 
nacre, just as the marginal region of the mantle. 
The chief difference between this sinus of Pinna and the 
pallial sinus of the Veneride, &c., seems to be this,—that in 
Pinna the shelly matter deposited within the pallial line is. 
nacreous ; that deposited without, fibrous ; whereas in the other 
bivalves the matter deposited within and without is of the same 
nature. The pallial sinus, therefore, in changing its place suc- 
