233 
instances, they secm to form for themselves), have all the 
characters of Modiola: even with respect to these, he is 
sometimes mistaken, for in several of the Fossil species he 
has described, what he calls the tube is no more thana 
solid mass of marley Stone, filling the hole in which the 
shell lies, enveloping it and preserving its form, while the 
Coral or substance the shell has bored into, has decayed 
away. I have specimens in several stages of decay, both 
foreign and English, with the impressions of the Coral 
upon their surfaces. Parkinson has figured similar ones, 
Vol. 11. t. 12. f. land 2, and in Vol. III. ¢. 14. f. 6 and 
12, and I have figured others in British Mineralogy, tab. 
$23, under the name Mytilus tunicatus. Delamarck seems 
to have had a right conception of the recent ones, such as 
Modiola curvirostra, Brit. Min. Vol. 3. p. 182, Mytilus 
lithophagus var. Linn. Trans. VIII. p. 270, ¢. 6, f. 2, 
and would perhaps have left them with the Modiola, had 
he not been misled by the Fossils. 
Figs. 1, 2, and 4, are from specimens selected from a 
large decaying mass of fossil wood, the hollows of which 
were filled with Marle; masses of wood thus preserved were 
found in great abundance in the Highgate Clay, towards the 
part on the southern side of the hill, where the Sand com- 
menced, and on exposure the wood became powdery, 
while the shells became more or less chalky, and were 
easily separated. Some few specimens in the medium state 
were obtained : the wood formed a sort of cylinder, and the 
animal’s broader ends were all around its centre, pushing, 
as it were, for the greatest share, the largest forwardest, and 
so on in succession, to those not so big as a barley-corn 
filling the interstices, the intervening ones often having two 
or three protuberances, like rollers or cushions of defence, 
(fig. 4) to keep them from the larger ones, and sometimes 
the larger ones seemed to have impressions in their sides 
corresponding with them, as if obliged to give way; the 
