414 
Mr. Dillwyn on fossil shells. 
of these strata have the ligament external, and consequently, 
that internal ligaments were confined to the monomyairia, 
till after the lias had been deposited. 
In the secondary beds above the lias, all the shells may be 
referred to some of our now existing orders of animals, and 
the extinction of the unknown orders is immediately fol- 
lowed by the first appearance of another order of mollusca, 
to which Lamarck has limited the name of gasteropoda, and, 
as was first suggested to me by Mr. Miller, all those fossils 
of the older strata, which have been supposed to be inside 
and outside casts of patellae, were obviously formed in the 
concave sides of the vertebra, or by the intervertebral carti- 
lages of a fish. As a few of the carnivorous trachelipoda 
are said to have been found in the oolites, their first appear- 
ance may probably be referred to the same epoch ; but I have 
not myself been able to detect either of the families of this 
section of trachelipoda in any secondary bed, excepting the 
denuded tracts of green sand in Devonshire ; and there, per- 
forations exactly similar to those which abound among ter- 
tiary and recent shells are also of frequent occurrence, 
although I have never met with any such perforation in any 
other secondary formation, nor even in any of those regular 
beds of green sand, which actually underlie the chalk in other 
counties. I am not enough of a geologist to decide, as to 
whether any admixture of secondary and tertiary fossils may 
possibly have taken place when these denudations were 
made, but I can in no other way account for the fact, that all 
the species which have been perforated, as well as the carni- 
vorous trachelipodes themselves, are nearly similar to those 
of the London clay ; and I have never been able to find any 
