84. Dr. E. von Martens on the Subdivisions of Pinna. 
cessively with the imcrement of the whole animal, leaves, in the 
other bivalves, no trace behind; but in Pinna the difference of 
the shelly matter indicates its position in all preceding stages 
of growth, as far as the matter formed in this stage is not quite 
worn off. 
There is a difficulty which I am at present unable to explain : 
in some specimens the outside fissure is visible not only in the 
denuded nacreous matter, but extends also over a part of the 
outer fibrous strata. I have seen only few of such specimens, 
but among them one of extraordinary size and thickness, be- 
longing to Pinna rugosa, Sow. 
The presence or absence of a sinus is constant, so far as I 
could ascertain, in all specimens of the same species. Species 
nearly resembling each other in other characters and in the 
whole habit, agree also in most cases in respect to the sinus. 
The external sculpture and the outlines of the whole valve 
being changed often with age, the sinus remains in each stage 
of growth. It is visible at first sight at the inside; and in 
most cases its presence may inferred from the outside of the 
apical region. I therefore consider it a good character for the 
systematic arrangement of the species ; and such an arrangement 
will prove a natural one. The species provided with a sinus 
may be called Pinne fisse, and are illustrated by Chemnitz’s 
above-cited fig. A, by Poli’s nobilis, Lamarck’s fossil Pinna sub- 
quadrivalvis, and the figures 8, 9, 20, 25, 26, 31, 50 of Reeve’s 
monograph. The species without sinus, Pinne integre (see 
Chemnitz’s fig. B), correspond to Adams’s definition of Atrina ; 
but in the list of species given in the ‘Genera’ either one or 
two species provided with a sinus have crept in, or the specific 
names are applied to others than those in Reeve’s subsequent 
monograph. In some of the Pinne integre the limitation of the 
nacreous portion forms a nearly straight line, as in P. sub- 
viridis ; 11 many others it forms an obtuse angle near the inner 
side of the muscular impression—a first step to the sinus of the 
other section (P. truncata, P. vexillum, &c.). 
Pinna saccata, L., so remarkable for its twisted form, differing 
in each individual, constitutes a third section, the nacre being 
very reduced, occupying only the muscular impressions and a 
small region between the larger muscle and the ligamental 
(dorsal) margin of the valve, while it is almost entirely absent 
on the opposite (ventral) part of the valve. But as the fibrous 
portion in this species is more smooth and more glossy than in 
the rest of the genus, the difference between both 1s at first 
sight not so striking. There is no fissure on the outside, and 
the apices are generally not worn off. While the other Pinne 
stand erect upon the ground, the apices being imbedded in 
