MOLLUSCA (except CEPHALOPODA). 137 



repetition of outward form at different epochs has already Gallery- 

 been noticed among the Brachiopoda (p. 115). Among VIII. 

 lamellibranchs from the same rocks are Alectryonia, Lima, 

 Gtenostreon, Inoccramus and the large Tricliites with its thick 

 shell, a favourite haunt of Lithodoimts, as various specimens 

 show. Another specimen shows a Litliodomns burrow in the 

 floor of the Oolitic sea, here formed of black Carboniferous 

 Limestone. Then follow many species of Trigonia, Astarte, 

 Pholadomya, Ceromya and other genera. Among all these 

 specimens may be noticed others from the Ironstone of 

 Duston, Northamptonshire, and from the Collyweston Slate. 

 In Yorkshire the Bajocian series includes beds of estuarine 

 origin, furnishing such forms as Unio and Anodon. 



The lamellibranchs from the Lias are arranged under Table-case 

 the three divisions of that marine formation : the Upper, of 

 Toarcian age, the Middle, of Pliensbachian, and the Lower, 

 of Sinemurian. Here one should notice Lcda ovum, which 

 gives its nauie to a horizon in the Upper Lias, Volsclla 

 [Modiola] scalprum from the Middle and Lower Lias, the very 

 familiar Grypliaea incurva, and the equally massive Hippo- 

 podium ponderosum, both from the Lower Lias (Fig. 72). 



14. 



Fig. 72. — Lamellibrancli shells from the Lower Lias. «, Hijjpoijodkmi 

 ponderosum ; h, Gryphsea incurva. Natural size. 



Oxytoma [Avicula] cyrjnipcs, from the Cleveland ironstone beds 

 of Yorkshire is a fine shell. The gastropods are all placed 

 together, since they are few in number, and nearly all com- 

 prised within three genera : Eucyclus, Cryptaenia, and Pleuro- 

 tomaria (Fig. 73). This last contains some large shells, those 

 of P. anglica being most numerous. 



Trias. The Mollusca come chiefly from the Rhaetic 



Table-case 

 14. 



