240 Mr. J. Morris on the Excavating Sponges. 
inhabiting the tubular and vesicular cavities in the shell of Pla- 
cuna sella, but uncertain as to what family it really belonged. 
The Prince of Canino, President, appointed a commission, con- 
sisting of Drs. Ruppell and Nardo and Prof.Géné, to express their 
opinions on the fact, and Dr. Nardo in their name made a report, 
from which the following remarks are abridged. ; 
The pecuharity described by M. Michelin consists in having 
noticed between the two faces of the superior valve of Placuna 
sella, on account of its transparency, a kind of arborescence with 
dichotomous and anastomosing branches, having the inferior 
branches thick and decreasing towards their extremities, which 
are generally sharp and forked. 
On the inner layer of the shell no pores were observed commu- 
nicating with the branches, but on the outer layer are numerous 
small perforations serially disposed and corresponding with the 
articulations. These cavities have been produced by a perfora- 
ting parasitic animal which has introduced itself into the sub- 
stance of the valve, and which in consequence of a greater resist- 
ance or hardness of the inner layer in contact with the animal of 
the Placuna, has been compelled to extend itself horizontally, so 
as to form the arborescence described. On some parts of the 
surface may be observed a few attempts at perforation which have 
been arrested by a new layer of solid matter. In the Milan city 
museum is a fine specimen of Placuna having both valves per- 
forated. The large size of the holes in this shell has allowed a 
portion of the animal filling the cavities to be carefully examined. 
It belongs to the class of sponges, and specially to the genus 
Vioa, which Dr. Nardo first described in his memoir on the per- 
forating sponges, published in the ‘ Annals of Science of the 
Lombardo-Venetian Kingdom*.’ From the form and arrange- 
ment of the siliceous spiculz, imbedded in the substance, sharp 
at one end and rounded at the other, it should be arranged (ac- 
cording to the system of Dr. Nardo) im the second order of sili- 
ceous sponges, the ninth family Viotde, and the first subfamily 
Vioina. This species appears to be distinct from all those pre- 
viously known and described, and may therefore bear the name 
Vioa Michelint. Dr. Nardo further adds as an important fact, 
that it is not only the Placune which have been attacked by this 
kind of sponge, but also univalve shells; and mentions a large 
specimen of Voluta in the Milan museum, which 1s perforated 
by a species of sponge distinct from the Vioa Michelini, as re- 
gards its mode of development, which although serial and den- 
dritic, has the vesicular and articulated cavities smaller and bored 
on both sides. 
Dr. Nardo concludes the report with some remarks relative to 
* See the volume previously quoted. 
