376 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 4, 



which in some cases in the vicinity of igneous rocks have acquired an 

 opahne character. 



I cannot conclude these remarks without expressing my regret that 

 the observations are so desultory and unconnected ; at the same time 

 I trust, that until we get the full result of M. Tchihatcheff's more 

 extensive discoveries, they will afford some clue to the complicated 

 geology of this part of Asia Minor ; at all events they show the ex- 

 tent to which the different formations have been disturbed by igneous 

 agency at various periods. I look forward with great interest to the 

 pubhcation of M. TchihatchefTs promised work. 



2. On Tylostoma, a proposed Genus of G aster opodous MollusJcs. 

 By Daniel Sharpe, Esq., F.G.S. 



Among the organic remains of the beds belonging to the cretaceous 

 system in Portugal are many casts of univalve shells, which have 

 certain common characters entitling them to be classed together and 

 distinguishing them from any described genus. Few of the specimens 

 found retain any portion of the shell, and in no instance was I for- 

 tunate enough to find the shell perfectly preserved, so that the 

 specific descriptions are necessarily imperfect ; but there are among 

 them materials to show the generic characters nearly complete. 



These shells are either globose or ovate, with a spire of moderate 

 elevation, and resembling in form either the GlobiconchcB or certain 

 Naticce and Phasianellce. The surface is nearly smooth. The mouth 

 is nearly semilunate, with the lips united in a regular curve ante- 

 riorly and meeting above at a sharp angle. The outer lip is thick- 

 ened internally by a callosity which reaches along its whole extent, 

 and which, in some of the species, is slightly toothed : this internal 

 callosity is repeated at regular intervals, which differ according to 

 the species, the most frequent repetition being twice, and the most 

 distant being once in a volution. The iimer lip is expanded over the 

 body-whorl, and almost conceals the columella, which is probably 

 solid. At the periods of forming the internal callosity, the form of 

 the mouth is temporarily modified by the outer lip being somewhat 

 constricted, and by the aperture being lengthened at its upper ex- 

 tremity by a gradual rising of the upper edge of the whorl. A little 

 in advance of the callosity the aperture returns to its previous form ; 

 the outer lip expands again to its former dimensions, and the top of 

 the whorl slopes down gradually to its former level. Thus, at each 

 of these periods of growth the shell presents an external constriction 

 of the whorl, an internal thickening of the whole extent of the outer 

 lip, and a temporary rising of the upper edge of the whorl. These 

 changes become more marked as the shells increase in age : in the 

 young state, the shells show such slight traces of these peculiarities 

 that they can scarcely be distinguished from species of Globiconcha 

 or Natica. 



The recurrent changes in the level of the upper edge of the whorl 

 give a peculiar unsymmetrical appearance to the spire, by which the 



