20 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



lines of growth towards the acute oblique apex, while in some 

 specimens another shallow groove proceeds parallel with the left 

 margin. Some radiating impressed lines are also observable in some 

 specimens. The inner or lower surface is convex aiid more or less 

 finely rugose or papillate, but no spiral or other lines of growth are 

 to be seen. It is surrounded by a raised rim, which is grooved at 

 its periphery. In texture it is evidently horny, as upon being tested 

 by Mr. Newton with acid no reaction took place, as would have been 

 the case had it been calcareous. These organisms appear very 

 abundant, as according to Fischer as many as 500 may be found in 

 a space of 7 or 8 square cm. Although as a rule thej'' are thus fouiul 

 congregated in the matrix without any shells being present, I have 

 found stray specimens associated with various freshwater shells, such 

 as Fhmorbis, Viviparus, and bivalves. The presence of one of these 

 structures in a piece of rock containing the shell of a species of 

 Viviparus or allied genus led me to compare it with some of the 

 Slavonian operculates, and I found that they accurately fitted the 

 aperture of Tylopoma PUari. It may, consequently, be reasonably 

 inferred that these structures are the operculum of a species of 

 Tylopoma or of some allied genus, on the following grounds : (1) They 

 bear no resemblance to the internal shell of any known mollusc. 

 (2) They have combined characters of the operculum of various opistho- 

 branchiate molluscs, i.e. they share the amorphous inner surface of 

 the operculum of Bdlujnia and the excentric lines of growth of the 

 outer surface of that of Fomatiaa. (3) They differ from the operculum 

 of Viviparus, which has concentric rings on both surfaces and lacks 

 the grooved rim. The fact that it has never been found in situ in the 

 aperture of a shell does not, in my opinion, militate against this 

 assumption, since amongst the numerous specimens of Viviparus and 

 allied genera from the Slavonian, Dalmatian, Croatian, and Roumanian 

 beds which have passed througli my hands I have never come across 

 a single specimen with the operculum in situ. This, I think, may be 

 accounted for by the fact that when these creatures died and the soft 

 parts perished the operculum would probably sink to the bottom and 

 the shell float away. Perhaps some day a specimen may come to 

 light with the operculum in situ, when all doubts on the subject will 

 vanish. 



