68 PALEONTOLOGY 



mobice and Mesodesmce occur in the greensand ; Donax and 

 Syndosmya in the eocene ; Gastrana, (= Vcnenqris, Lam.) and 

 Lutraria in the miocenc. Lutraria rugosa, still living on the 

 coast of Portugal, is fossil in the raised beaches of Sussex. 



The oldest forms of razor-fish (Solenidce) are those with 

 the transverse internal rib (Solccurtus), which occur in the 

 neocomian, whilst time Solens and Glycimcrvi appear first in 

 the eocene strata. The genus Mya, as now restricted to the 

 species resembling M. armaria, are only met with in the 

 newer tertiary. Corbida ranges upwards from the lower 

 oolites ; Nemra appears in the upper greensand ; and Thetis 

 (= Poromya, Forbes) in the neocomian. 



Above 100 species of Panopwa (a genus essentially like 

 Mya) have been obtained from oolitic and tertiary strata in 

 all parts of the world. They are with difficulty distinguished 

 from those equally numerous forms of Anatinidm which have 

 been associated with Pholadomya on account of the tenuity 

 of their finely-granulated valves ; they constitute the genus 

 Myacltcs (Bronn), and occur in all the palaeozoic and secondary 

 rocks ; some of the oolitic and cretaceous species are distin- 

 guished by V-shaped furrows (fig. 16, 6). Still more numerous 

 are the fossil forms of Pholadomya, which range upwards from 

 the lias, but are reduced to a single species now living in the 

 Caribbean seas. Shells with the umbones fissured like 

 Anatina also occur in the oolites. Pandora first appears in 

 the older tertiary. Amongst the extinct genera referred to 

 this family are the Silurian Grammysia (fig. 13, io), with valves 

 folded transversely ; the carboniferous Edmondia (fig. 13, n), 

 with large oblique cartilage plates ; and Gardiomorpha, shaped 



like Isocardla ; and tin litic Ccromya (Ag.), which also 



resembles the heart-cockle in form. Cercomya is an oolitic 

 Anatina, with the posterior end of the valves much attenuated. 



The genus Gastrochmna appears in the lower oolites ; and 

 easts of its burrows are frequently preserved after the decom- 



