780 



HISTORICAL GEOLOGY. 



taceous ; the beak is twisted to one side, as is implied in tlie name. Trigonia 

 (Fig. 1291), the name alluding to the somewhat triangular form, has over 

 100 Jurassic species. Another peculiar type common in the Middle Oolyte 



1288-1293. 



Lajeellibkaxchs. — Fig. 1283, Ostrea Marshii, Lower Oolyte ; 1289, Exogyra vii-gula, Kimmeridgian ; 1290, 

 Grypliaea dilatata, Callovian ; 1291, Trigonia clavellata, Corallian ; 1292, Astarte minima, Corallian ; 1293, 

 Diceras arietinum, Diceratian. 



in the northern Alps is that of Diceras (Fig. 1293), a species in which the 

 beak of each valve is curved spirally ; it is related to the modern Chama. Of 



existing genera having many Jurassic species there 

 1294. are Ostrea, Pecten, Lima (Fig. 1286), Astarte (Fig. 



1292), Lucina, Corhula, Nucula, Pholadortiya, and 

 many others. 



Gastropods were very numerous. The number 

 of species found in British Jurassic rocks alone 

 is nearly 1000; and of these over 10 per cent 

 were of the old genus Pleurotomaria, the number 

 being larger than for all preceding time. It was 

 the culminating time for the type ; only two living 

 species are known. Other genera of many species 

 dating from the Paleozoic, and also modern, are 

 Trochus, Turbo, Patella, Natica, which comprise 

 25 per cent of the British Jurassic Gastropods ; 

 and among the many of Mesozoic origin, Cerithium has 10 per cent of all the 



Gastropod. — Fig. 1294, Neri- 

 nea Goodhallii, Corallian. 



