DAVIDSON — PAL^ONTOLOGICAL NOTES ON THE BRAOHIOPODA. 113 
in a similai- manner to oysters, scallops, and Spondylus slriatiis, the 
lamelljc and spines serving to retain the animal in a fixed position ; 
but Professor Koninck objects, that these spines are often so long and 
so delicate as to make one believe that"they would be fractured under 
such conditions. In a paper communicated in 18.53 to the Geological 
Society, I endeavom-ed to show that tlie ventral valve in Chonetes 
comoides, Producta heinisphcerica, F. gigantea, &c. is from four to 
eight times thicker, especially near the middle, than the dorsal one, 
which is, on the contrary, thin and lightj and thus if the animal 
had lived with its larger and ponderous valve uppermost, no muscular 
power which it could have exercised would have been, in all pro- 
bability, sufficient to raise the ventral valve ; while, on the contrary, 
supposing the shell to have rested on its larger or ventral valve, the 
slightest force would suffice to raise and separate the smaller or 
dorsal valve. 
Some singular forms of Producta, such as P. proboscidea, P. genuina, 
&c. have their ventral valves prolonged for more than two inches 
Lign. 5. — Longitudinal Section ot Chonetes comoides. 
beyond the dorsal, the edges being rolled together in the shape of one 
or two tubes (PI. 4, figs. 1 and 2). This circumstance has led 
d'Orbigny to explain this singular appearance, by supposing that the 
animal, from having lived in cavities, or half buried in mud, was 
obliged to prolong the edge of its mantle, and consequently also of its 
shell, so as to reach the surface of the sea-bed for maintaining the 
brachial currents. Mr. S. P. Woodward suggests that the shell of 
some species may perhaps have been attached by a peduncle when 
VOL. II. I 
