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radiating undulations with numerous minute aculei when 
most perfect ; but these are sometimes wanting : the other 
valve is for the most part externally concave, without 
undulations, but bearing sharp spines, winch are hol- 
low, and especially at their points pressed close to the 
surface and often hooked* : the surface that received the 
half external cartilage is sometimes very distinct, ex- 
tending transversely upon the beaks; so as almost to 
form ears. It is a thinner shell than others of the genus. 
Mr. Parkinson has made a new genus of this shell ; the 
four teeth in the hollow valve, and the entire edge not 
agreeing with La Marck’s definition of the genus 
Plicatula, to which, however, it belongs. I have re- 
ceived many French specimens of it from my generous 
friend, Mr. de Gerville, and also of another species 
almost free from spines ; they were named by Mr. de 
France, Plicatula spinosa and P. rarispina. The first 
British specimen I have seen, fig. 4, was lent me several 
years ago by J. L. Baker, Esq. of Stouts Hill, near 
Lley, Gloucestershire, where it probably was found ; it 
is of a light colour and satiny lustre, with dark coloured 
ermine-like spots, where the spines project from the sur- 
face, that give it a peculiar richness. 
Fig. 1 is from an excellent specimen found by the Rev. 
W. D. Conybeare at Branston Tunnel, Northampton- 
shire : in the upper beds of Lyas Clay, fig. 2, are two 
views of the flatter valve, from the same place. Mr. 
Conybeare has shewn me a part of one valve two inches 
long, from the tunnel at Crick, Northamptonshire. I 
suspect this belongs to another species. 
Mr. Parkinson’s specimen was found three feet 
below the surface at Leonard Stanley, in Gloucestershire, 
(also upper Lyas.) 
Fig. 3 is from a French specimen, which is rather 
larger than usual. 
It frequently happens, among the French speci- 
mens, that the flatter valve is attached to a Belemnite, 
which appears as if water worn. Belemnites are also 
found in the same stratum with the Plicatula in England, 
and may help to identify the French stratum. 
* Parkinson has made this a part of his generic character, not knowing 
of a species without spines, which occurs in France. 
