306 Miscellaneous. 
somewhat among the elements of the subjacent tissues (muscular 
fibres and conneccive cells). It is thus that, by successive 
appositions of new parts issuing from the pedal fissure, the striz 
are formed, of which only the most salient are visible to the naked 
eye or the lens. 
On the other surface of the operculum we must distinguish two 
regions, the surface of insertion of the columellar muscle, and the 
free internal surface. It is very easy to see that the free inner 
surface and the outer surface of the operculum have not the same 
constitution. The inner surface is covered with a homogeneous layer 
which forms, as it were, a varnish without any strie perceptible to 
the eye. This coating may even be so thick that we cannot see 
through it the striz of the other surface (Murex). In other cases 
it is delicate enovgh not to hide them (Littorina, Trochus). This 
difference of constitution arises from a difference of origin. In front 
of the surface of insertion the metapodinm forms an anterior expan- 
sion or lip, which, during the life of the animal, is constantly applied 
against the inner unattached part of the operculum. The epithelial 
cells of this anterior lip produce the varnish. 
It still remains to ascertain why the opercular material is rolled 
into a spiral. To elucidate this point we must study the muscular 
impression. As the operculum grows, the surface of insertion of 
the operculum is displaced with a slight movement of rotation, 
since during the same time the shell grows in a spiral. The 
muscle attaches itself to the newly formed parts, abandoning the 
old parts on the side of the parietal margin. These stages of the 
columellar muscle are marked by strixw independent of those of the 
superior surface. We observe them when preparing an operculum 
after removing all traces of muscle. By studying these lines we can 
even understand why there are opercula of which the form always 
remains the same, while there are others of which the form varies 
with the age of the animal. The latter are said to have a nucleus 
of formation. The posterior secreting portion of the foot always 
retains the same form in tlfe first case, whereas in the second we 
see it from nearly circular become almost straight. 
Thus we see that the operculum is a production of a definite 
portion of the epithelium of the foot, and appears to be very 
different from the byssus of the Acephala, which is produced by a 
highly developed gland occupying a good part of the volume of the 
foot. It is still more different from the second valve of a shell.— 
Comptes Rendus, January 28, 1884, p. 236. 
A Fungus infesting Flies. 
Prof. Leidy directed attention to a vial filled with flies adherent 
to fragments of leaves. He stated that on the Ist of August, the 
last summer, he had noticed that from the swarm of flies that 
were attracted by the ripe fruit of a black mulberry, Morus nigra, 
many settled on the underside of the leaves, and there became fixed 
and died from the invasion of a fungus, in the same manner as the 
