412 Capt. Brown’s Monograph on the Pisidium. 
ErRBsMUSCHEL.—Pisidium. 
Animal.—Instead of the tubular trachea, it has a narrow fleshy 
projection next the fore-part of the shell: the foot long and thin. 
Shell.—Oblong, sides unequal, completely closing: in the right 
valve one, and in the left valve two opposite, very small primary 
teeth: behind and before, two thin lamellar side teeth ; those of the 
latter cleft in the right valve, in order to receive the opposite ones. 
Pfeiffer remarks if this genus is not viviparous, as is the case with 
the Cyclas, then there would be a still more marked difference be- 
tween the two. He says he never succeeded in finishing an em- 
bryo in the open shells, nor in obtaining young ones from the living 
animal ; and, therefore, concludes that they are oviparous. 
From the specimens of the Pzszdium obtusale, kept alive by 
me, I have ascertained that they are not oviparous, as Pfeiffer 
supposes, having produced their young alive, since I got them; 
so that the animals of this genus are undoubtedly viviparous. 
The young shells are much more flat than the adults, in propor- 
tion to their size; which, at first, is so exceedingly small as to be 
scarcely perceptible to the naked eye, their colour being of a pale 
yellowish white: they have only yet attained about half the size of 
the parent shells. 
Since writing the above, I changed the water in the glass, (6th 
February,) and rendered it tepid, in order to induce the animals to 
protrude themselves. While in the act of watching them, I found 
that the favourable temperature brought on parturition ; and I 
witnessed several of the animals produce their young in a perfect 
state. This operation was performed by the valves of the shells 
being opened, and then suddenly brought together with a kind of 
jerk, when the foetus was ejected from between them to a little 
distance from the parent shells. ‘These young shells are somewhat 
larger than those produced about three months ago. _ 
There is a considerable difference in the animals of the Cyclas 
and Pisidium ; those of the former having a shorter and thicker 
foot in proportion to the size of the animal, and having a double 
united tube at the posterior termination of the cloak, whereas that 
of the latter is a single tube ; which will be better understood by. 
the following figures, giving a representation of the manner in 
which the animal performs locomotion. Fig. I. is the Péisidium, 
considerably magnified, and Fig. II. the Cyclas. 

Fig. II. 
1. Diz Scurere ErssmuscHeL.—Pisidium obliquum. 
P. testa ovata, oblique trigona, ventricosa, cornea, striata, um- 
bone obtuso. 
