LAMELLIBEANCHIATA. 



69 



all the palaeozoic rocks, is very thin and concentrically fur- 

 rowed (fig. 17, *)• Many other genera have been proposed whose 

 characters are even more imperfectly understood. Monotis 

 (Salinarius) one of the common shells of the trias, has no 

 anterior ear. Pteropema (Lycett), an oolitic form, has a 

 winged shell, with numerous small anterior teeth and long 

 posterior laminae. The genus Gervillia (fig. 18, 4), ranging 



Fig 18. 

 Secondary Bivalves. 



- 1. Gryphsea arcuata, Lam. ; Lias, Charmouth. 



2. Pecten (Neithea) quinquecostata, Sby. ; Chalk, Sussex. 



3. Pulvinites Adansoni, Defr. (internal mould) ; Corallian, Rochelle. 



4. Gervillia anceps, Dh. ; L. Greensand, Isle of Wight. 



5. Inoceramus sulcatus, Park.; Gault, Folkestone. 



6. Cucullsea (Macrodon) Hirsonensis, D'Arch. ; Great Oolite, Min- 



chinhampton. 



7. Isoarca cordiformis, Schloth. ; Corallian, Nattlieim. 



8. Myoplioria decussata, Miint. ; Trias, S. Cassian. 



from the carboniferous limestone to the chalk, consists of elon- 

 gated shells, with several cartilage-pits in the ligamental area. 

 BaTcewellia, found in the Penman, has an anterior muscular 

 impression like Area. The recent genus Perna commenced 

 in the lias or preceding formation, and exhibits great variety 

 of shape. Pulvinites Adansonii (fig. 18, 3) appears to have 

 been a Perna with a byssal foramen like Anomia; and Inoee- 



