PHILIPPI S COMPARATIVE TABLES OF MOLLUSCA. 9 



Venus geogra'phica^ V, Iceta, Poli, Turbo neritoides, &c., did not 

 exist during the tertiary period. 



It may be stated as a general rule, that the differences observable 

 between recent and fossil individuals of the same species are not 

 greater than we find in the case of recent specimens, so that we may 

 often be in doubt whether a particular specimen is fossil or not. This 

 is especially the case with shells obtained from the clay of Abbate, 

 near Palermo, which are washed out by the sea, and are often taken 

 to serve as the temporary habitation of the hermit crabs. These 

 species are often remarkably well preserved, and there is some ex- 

 cuse for their being occasionally mistaken for recent specimens. 

 This has happened, for instance, to those conchologists who, with 

 Linnaeus, have described Dentalium elephantinum as a Sicilian spe- 

 cies ; and also in the case of M. Kiener, who represented Murex 

 vaginatus as living ; and it is a matter of serious complaint that he 

 allowed himself to change its name. 



It is notwithstanding worthy of note, that several species seem to 

 have attained a larger size in former periods than at present, and 

 this is especially fhe case with regard to Lucina radula, L.fragilis, 

 Cytherea rudis, Poli, Venus radiata, Cardium Deshayesii, C. pa- 

 pillosum, Mytilus edulis, Pileopsis ungarica, Turritella communis, 

 and T. triplicata. This list indeed I could increase still further, (in 

 my ' Enumeratio' I have always given the dimensions of the fossil 

 and living forms when they exhibited any marked difference,) but 

 the greater number of the species agree accurately in point of size, 

 while it is remarkable that some, though but few, were smaller du- 

 ring the tertiary period than they are at present. This is the case 

 with Bulla lignaria and Terebratula vitrea, which formerly did not 

 attain half the size that they do now ; and next to them we may 

 mention Corbula nucleus. One can hardly come to any other con- 

 clusions from these facts, than that formerly the conditions with re- 

 gard to number, locality, sea-bottom, &c. were more favourable for 

 the development and growth of some species, and less so with regard 

 to another and much smaller group ; but that in general these condi- 

 tions were very similar to those that obtain at present. 



4-. The Distribution of Species, and considerations tvith regard to 

 Subdivisions of the Tertiary period. 



The fossils are for the most part principally abundant in clay, marl 

 and shelly sand ; but it is of no consequence in the present discussion 

 what may be the lithological character of particular localities, since 

 the same species occur in clay and shelly sand, and even in com- 

 pact limestone, — as may be seen, for example, very clearly near 

 Palermo, — just as at present we find, on the whole, the same species 

 on a sandy and muddy part of the coast. Moreover, the geok)gical 

 relations, so far as Sicily is concerned, have been described most 

 fully by the late M. Hoffmann*; and the relations of the tertiary 



* See his ' Geognostische Beobachtungen gesaramelt auf einer Reise (lurch 

 Italien und Sicilien,' Berlin 1839. 



