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. Riippell and Nardo and Prof. Gene, to express their 

 opinions on the fact, and Dr. Nardo in their name made a report, 

 from which the following remarks are abridged. 



The peculiarity 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 spiculse, 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) in the second order of sili- 

 ceous sponges, the ninth family Vioidce, 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 Michelini. Dr. Nardo further adds as an important fact, 

 that it is not only the Placuna 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 is 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 

 * Sec the volume previously quoted. 



