498 



LAMELLIBRANCHIATA OF TERTIARY AGE. 



The Wealden beds first being in Unio, Cydas, and Cyrena. The 

 Neocomian seas introduced Pedunculus, Spondylus, Crassatella, Thetis, 

 Mesodesma, and Solecurtus. The Upper Greensand introduces Cre- 



Fig. 126. Cardium. 



Fig. 127. Trigoiiia. 



nella, Chama, Capsula, Machaera, Clavagella. The Chalk makes 

 Vulsella known. The following genera come in the Lower Tertiary 

 period Nucinella, Lithocardium, Cryptodon, Diplodonta, Pythina, 

 Petricola, Psammobia, Sanguinolaria, Semele, 

 Syndosmya, Donax, Solen, Potamomya, Pandora, 

 Pholas, Teredina, and Cardilia. The Middle 

 Tertiary makes known Artemis, Trigona, Luci- 

 nopsis, Tapes, Venerupis, Lutraria, Gastrana, 

 Mya, Glycimeris, Yoldia, Solenella, Tridacna. 

 Very few genera appear in the Upper Tertiary 

 strata which were not previously known. The 

 Fig. 128. Phoiadomya. range in time of genera is always being carried 

 further back, and needs to be considered by the 

 student as affecting the locality or district under consideration ; 

 for the object of all collections of fossils is to demonstrate the local 

 geographical distribution of life in geological time. The use of a 

 summary consists in the evidence it furnishes of the change of life 

 in time, by indicating epochs when new genera made their appear- 

 ance. It is only, however, after comparison of the later forms with 

 the types with which they have family affinities, that we recognise 

 the evolution which they represent. 1 



Gasteropoda. So far as the palaeontologist is concerned, the soft 

 Gasteropoda which have no shells may be disregarded, and we may 

 define the class palaeontologically as characterised by having the body 

 more or less perfectly contained in a shell which usually consists of 

 one piece, with the aperture often closed by a horny or shelly oper- 

 culum. The shell may be tubular, as in Dentalium, conical, as in 

 Patella ; but more commonly exhibits some degree of spiral growth. 

 Comparatively few genera are absolutely extinct. Murchisonia is 



1 See "Structural and Systematic Conchology," by George W. Tryon, Phila- 

 delphia, 1882. Also Woodward's "Manual of' the Mollusca," of which it is 

 a new edition. 



