GASTEROPODA 69 



position of the coral in which they were made. Clavagella 

 dates from the upper greensand, and Asycrgillum from the 

 miocene. Saxicava is found in the newer tertiary and raised 

 beaches of Northern Europe ; and the great species commonly 

 called " Panojxza " Norwegim is a characteristic fossil of the 

 newer pliocene of Britain and Greenland. 



The Pholades and ship-worms appear first in the oolitic 

 strata. Forms resembling the recent Martesia striata have 

 been discovered in fossil wood of the lias and Speeton clay. 

 Jouanneiia (Desm.) was first known as a miocene fossil ; and 

 Pholas occurs in the older tertiary. Extinct species of Teredo 

 are found in the silicified wood of the greensand of Blackdown 

 and in the fossil palm-fruits of Brabant and Sheppy. The 

 drift-wood of the London clay is usually perforated by the 

 ship-worm, and also by an oxtinct form (Teredina, fig. 16, 9), 

 which resembles Martesia in possessing an umbonal shield : 

 when adult it not only closes the anterior pedal opening, but 

 also cements its valves to the shelly lining of its burrow, like 

 an Aspcrgillwm. Specimens have been obtained in which the 

 whole interior of the valves and tube had been excessively 

 thickened towards the close of life by successive layers of 

 shell. 



Class III.— GASTEROPODA. 



Fossil univalves — the remains of spiral and limpet-like 

 shells — are not wanting in any but the very oldest fossili- 

 ferous rocks ("lingula flags"). From the lower Silurian, 

 where less than 100 species, referable to scarcely more than 

 ten genera, are found, they increase in number and variety 

 slowly and regularly up to the newer tertiaries, which have 

 afforded ten times as many genera and twenty times as many 

 species. The total number of fossil marine univalves is less 

 than 6000 ; the recent exceed 8000 ; and although we may 

 expect to discover more new fossil species than recent, yet it 



