MOLLtrSCA (except CEPHALOPODA). 
137 
repetition of outward form at diflerent epoclis has already Gallery 
been noticed among the Brachiopoda (p. 115). Among VIII. 
lamellibranchs from the same rocks are Alectryonia, Lima, 
Ctenostrcon, Inoceramus, and tlie large THchitcs with its tluck 
shell, a favourite haunt of LitJwdomiis, as various specimens 
show. Another specimen shows a Lithodovnis burrow in the 
door 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 Yorkshii'e the Bajocian series includes beds of estuarine 
origin, furnishing such forms as Unio and Anodonta. 
The lamellibranchs from the Lias are arranged under Table-ease 
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 name to a horizon in the Upper Lias, Volsclla 
\ALodiola'\ scalp'um from the Middle and Lower Lias, the very 
familiar Griyphaea incurva, and the equally massive Hijypo- 
podmm 2 )onderosiim, both from the Lower Lias (Pig. 72). 
b 
Fig. 72. — Lamollibranch shells from the Lower Lias, a, Jlippopodinm 
ponderostan ; b, Gryphxa incurva. Natural size. 
Oxytoma [Avimla] cygnijws, 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, Cryptacnia, and Plenro- 
iomaria (Fig. 73). This last contains some large shells, those 
of r. anglica being most numerous. 
Trias. The Mollusca come chiefly from the Rhaetic 
