204 
guished by having a notch about the middle of the ventral 
margin and by one of the valves being flatter than the other. — 
Shells having the form of Pecten are founu in abundance, 
and about half a dozen kinds have been made out, though at 
present these cannot be referred to their species, the characters 
which distinguish them being of a different nature from those 
found in the adult shells ; for, contrary to what is usually the 
case, the Pectens grow to a considerable microscopic size before 
the proper sculpture begins to be formed, and this intermediate 
growth resembles neither the foetal shell nor the adult in 
structure. All the young Pectens which have been observed 
have a rounded shape like P. opercularis, and in all of them 
the nucleus is smooth, glassy, and transparent, with the valves 
ventricose and obscurely triangular, like those of Anomia. 
One form differs from all the others in having the intermediate 
growth entirely composed of cellular structure, which might 
lead to a suspicion that it is Avicula, but the general character 
is strictly that of Pecten. As might be expected from the 
conditions under which the specimens are found, none are 
double, and it may be worth mentioning that those with a 
cellular structure are flat, and, as far as has yet been observed, 
are all the same valve. The other forms are convex, and 
have the external surface pitted over with more or less numer- 
ous deeply excavated depressions of various forms, differently 
arranged so as to make so many distinct patterns . — Lima sub- 
auriculata is not uncommonly found of microscopic size, and 
it agrees with the characters given of the adult, excepting 
that the strong middle line is externally a furrow and inter- 
nally a ridge, instead of the reverse. In this shell no dis- 
tinct line of demarcation is seen between the foetal shell and 
the aftergrowth. — Crenella presents an unusual case, as 
shown by the series of mounted specimens, in which the 
growth is seen to be continued to a considerable size on the 
plan of the original foetal shell, and then suddenly changes 
to a totally different character, as has been already observed 
