244 
Avicula decipiens, L. G. de Koninclc. 
PI. XXir, Pig. 13. 
Shell of the same size as the preceding, siiboval, elongated, nearly 
twice as long as broad, and slightly ineqnivalved. Although the valves 
are shallow, their large and triangular posterior wing is distinctly marked 
and strongly depressed. The small, non-prominent beaks are placed at a 
short distance from the anterior margin. The anterior ears are indistinct. 
The surface was covered with indistinct, irregular, concentric undulations. 
The imj)ression of the adductor is rounded, relatively large, and posterior. 
Dimensions. — The length is twenty-one, breadth fourteen, and thick- 
ness seven millimetres. 
Delations and Differences, — It resembles the j)receding species in size 
and anterior ornaments ; but differs by its relative thinness, and by the much 
less anterior position of its beaks. 
Horizon and Locality. — It accompanies ^4. intmnescens in the blackish- 
grey limestone at Harper’s Hill. I have seen but one specimen of it. 
Avicula intumescens, L. G. de Koninclc. 
PI. XXIT, Pig. 12. 
This is a rather small, subequivalved, sub trigonal, gibbous, oblique 
shell, longer than broad. It is much broader posteriorly than on the opposite 
side. Its posterior ear is depressed, triangular, and well developed, and is 
attached to the rest of the shell by a rather abrupt slope. The beaks are 
anterior and thick. The ventral margin is slightly sinuous. The right valve 
is rather smaller and shallower than the left. The cast has ^n’eserved traces 
of the thin concentric lamellm with which tlie surface was probably covered. 
The impression of the adductor muscle is small and close to the beaks. 
Another much smaller imi^ression is found in the hollow of the beak, and 
served as the point of attachment for the byssus. 
Dimensions. — The length is twenty-two, breadth sixteen, and thickness 
thirteen millimetres. 
