LAMELLIBKANCHIATA. 77 



car diet) are found in the secondary and tertiary strata; of the 

 50 recent forms, one only is Arctic, and this occurs in the 

 glacial deposits of England. The allied genus Myoconcha 

 is characteristic of the older secondary rocks, and Hippo- 

 podium of the lias. 



The Veneridce are pre-eminenly characteristic of the tertiary 

 and present period. Some obscure species of Venus are found 

 in the oolites : better marked species of Cytherea occur in the 

 greensands ; Artemis, Tfigona, Lucinopsis, Venervpis, and 

 Tapes appear in the middle tertiary ; Petricola in the eocene. 

 The only extinct form is Grccteloupia (fig. 20, 8), which differs 

 but little from Trigona. 



The Mactras and Tellens are also comparatively modern 

 groups ; most of the supposed oolitic species belong to the 

 Lucinid-ce, except Sowerbya (fig. 20, 5), which has a pallial sinus, 

 and is found in the oolites of Malton and Portland. Psam- 

 mobice and Mesodesmce occur in the greensand; Donax and 

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

 Lutraria in the miocene. 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 (Solecurtus), which occur in the 

 neocomian, whilst true Solens and Glycimeris appear first in 

 the eocene strata. The genus Mya 9 as now restricted to the 

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

 newer tertiary. Corbula ranges upwards from the lower 

 oolites; Necera appears in the upper greensand; and Thetis 

 (=Poiv?nya, Forbes) in the neocomian. 



Above 100 species of Panopcea (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 Anatinidce which have 

 been associated with PJiolccdomya on account of the tenuity 

 of their finely-granulated valves; they constitute the genus 



