FOSSIL SHELLS. 7 



LETTER III. 



TO THOMAS PENNANT, ESQ. 



THE fossil shells of this district, and sorts of stone, such as 

 have fallen within my observation, must not be passed over in 

 silence. And first, I must mention, as a great curiosity, a 

 specimen that was ploughed up in the chalky fields, near the 

 side of the Down, and given to me for the singularity of its 

 appearance, which, to an incurious eye, seems like a petrified 

 fish, of about four inches long 1 , the cardo passing for a head 

 and mouth. It is in reality a bivalve of the Linnaean genus of 

 my til-us and the species of cristagalli; called by Lister, rastellum ; 

 by Rumphius, ostreum plicatum minus; by D' Argenville, amis 

 porci, crista galli; and by those who make collections, cock's 

 comb.* Though I applied to several such in London, I never 

 could meet with an entire specimen ; nor could I ever find in 

 books any engraving from a perfect one. In the superb museum 

 at Leicester House, permission was given me to examine for 

 this article ; and, though I was disappointed as to the fossil, 

 I was highly gratified with the sight of several of the shells 

 themselves, in high preservation. This bivalve is only known 

 to inhabit the Indian Ocean, where it fixes itself to a zoophyte, 

 known by the name gorgonia. 



Cornua ammonis are very common about this village. As 

 we were cutting an inclining path up the Hanger, the labourers 

 found them frequently on that steep, just under the soil, in the 

 chalk, and of a considerable size. In the lane above Well- 

 head, in the way to Emshot, they abound in the bank in a 

 darkish sort of marl ; and are usually very small and soft ; but 

 in Clay's Pond, a little farther on, at the end of the pit, where 

 the soil is dug out for manure, I have occasionally observed 

 them of large dimensions, perhaps fourteen or sixteen inches 



'* Ostrea carinata, or keeled oyster, of Lamark. It is met with in the 

 department of Sarthe, and other places of France. The author is mis- 

 taken in supposing that this species is found in a recent state. It has been 

 satisfactorily proved, that there are no living species of those fossil shells 

 discovered in the old limestone formations, although there are some 

 existing individuals nearly allied to them. 



Petrifactions occur in three states ; sometimes they are a little altered, 

 sometimes they are converted into stone, and at other times the impres- 

 sions only of them, or the moulds in which they have been enclosed, 

 remain. ED. 



