DAVIDSON — PAL^ONTOLOGICAL^OTES ON THE BRACHIOPODA. 113 



in a similar manner to oysters, scallops, and Spondylus striatus, the 

 lamellae 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 1853 to the Geological 

 Society, I endeavoured to show that the ventral valve in Chonetes 

 comoides, Froducta hemisphcerica, P. gigantea, &c. is from four to 

 eight times thicker, especially near the middle, than the dorsal one, 

 which is, on the contrary, thin and light ; 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 j 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 of Chonetes comoides. 



beyond the dorsal, the edges being rolled together in the shape of one 

 or two tubes (PL 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 



