386 NATURAL SCIENCE. dec. 



of no less than 282 species of Mollusca, of which 45 per cent, are 

 Gastropoda. Also in the Blackdown Hills, at the base of our own 

 Upper Cretaceous series, univalves play a most important part, but 

 with marked changes of genera, forcing upon us the conclusion that 

 even before the Cretaceous period minor faunal provinces were clearly- 

 marked out. Of the forms here present, I'urbo is prominent, Solannm, 

 Natica, and Littorina are important, while Voluta, Conns, Strombiis, 

 Mitra, and PJiorns are entirely absent. We are thus led to the sugges- 

 tion that the conditions in our own region were probably of a more 

 temperate character than were those which prevailed further to the 

 south on the Austro-Saxon frontier. The answer to our first question 

 appears to be that the principal genera of univalve Mollusca in 

 Cretaceous times were identical with those of the present day. The 

 variations observed in their distribution are probably due to the 

 difference of temperature and surrounding conditions, while their 

 specific richness indicates that theirs was no new appearance, but a 

 direct continuity, obeying laws which research and comparison may 

 lead us to determine more fully in the future. 



Exclusive of molluscan genera now extinct, such as Inocevaimis 

 and Gervillia, I have been able to obtain the record from Germany 

 and England of thirty-three genera of Gastropoda embracing no less 

 than 171 species; and of bivalves (Pelecypoda) forty-eight genera 

 with close upon 300 species. 



For the extension of the Mollusca in the German Cretaceous, 

 one may refer to Geinitz, " Quadersandsteingebirge in Deutschland " 

 (1849-50), and F. A.Roemer, "Die Versteinerungen des Norddeutschen 

 Kreidegebirges " (1840), in both of which are to be found useful tables 

 indicating clearly the genera, the number of species, and their 

 general distribution in the Upper Cretaceous strata of Germany. 



2. In natural sequence to the above we are now led to inquire 

 whether at the present day any particular distribution is to be 

 observed in the Gastropoda on the one hand, and the Pelecypoda 

 (Lamellibranchia) on the other, more especially in connection with 

 their bathymetric extension. 



The records of the "Challenger" Expedition reveal the fact 

 that the Gastropoda rapidly diminish both in number and specific 

 richness as greater depths are approached, only comparatively few of 

 their genera having been met with beyond a limit of 500 fathoms. 

 Thus, of the genera known as occurring in the Upper Cretaceous, 

 twelve only have been found at depths exceeding 300 fathoms. Five 

 only appear to be of a decidedly ubiquitous character, viz., Trochus, 

 Actaon, Pletirotoma, the Scaphopod Dentalium, and more rarely Natica. 

 At a depth of over a thousand fathoms Fusiis, Scalana, Eidima, Miirex, 

 and Aporrhais have each been once met with, these same genera, 

 together with Solarium and Turbo, being more abundant between 300 

 and 1,000 fathoms. 



An examination of the records of the Lamellibranchiate genera 



