44.2 _ CRETACEOUS GASTROPODA 
in the ciliated fringe of the disk of the foot, but the shells are readily distinguished 
from it by the entire margin of the posterior end : this distinction also applies as 
regards Dentalium, but the separation from <Antale is more difficult, being 
apparently restricted to the more truncated shape of the posterior end in the present 
genus, and to a more hyaline structure of the shell. There are a large number of 
fossil, smooth species known, being distinguished by a remarkably thin shell, but 
until the exact form of the posterior end has in each special case been closely 
investigated, so as to ascertain whether the shell is at this pomt perfect or not, 
it would be of no use to propose any transfers of the species. 
These seven generic divisions as here distinguished represent, I believe, only 
the principal types, and it is very probable, as I had repeatedly occasion to notice, 
that the number of genera must soon be considerably increased. In all the recent 
species, so far as their animals have been observed, the foot is more or less cylindrical, 
adapted for digging in the sand, and consequently they all belong to the sub-order 
ScapHoropa. It will, however, be very desirable to examine also carefully some 
of the Preropopa with similarly formed shells. Supposing that the animals of 
some of these shells are identical in organization with Dzwrarup«, but that their 
foot is provided at its termination with wing-like lappets, which are better adapted 
for swimming than for creeping or digging, there must be a new sub-order for 
these shells formed in the ProsopocrpHaLa. I greatly suspect that this will 
probably have to be done with some of the species at present referred to Cresis, 
Cleodora and Vaginella, being remarkably thin shells, but apparently differing from 
typical species of these genera by having the posterior end pierced by a minute 
hole. 
Of recent species of Dzwrazip# there are about sixty known; they are 
found in all seas, though not very numerously represented, chiefly living on sandy 
or muddy ground between five and one hundred fathoms. The number of fossil 
species will amount to about one hundred. The first species are usually quoted 
from the devonian, but there are several silurian forms known which can hardly be 
excluded from this family, and it besides remains to be ascertained whether all the 
species described under Hyolithes are truly Pteropodous shells. The tertiary 
species are more numerous than those from any of the other formations. 
In giving a list of the species which have been described from cretaceous 
deposits I shall first mention those noticed by Pictet and Campiche in the “ Paléont. 
Suisse,” 83me. Ser., pp. 725-728, and then note any further additions made since 
the publication of that memoir. 
1. Dentalium valangiense, Pict. et Camp., is probably a species of Antale. 
Dent. cylindricum, Sow., is apparently also an Antale, but it may belong to Gadila. 
83. Dent. decussatum, Sow., is a true Dentalium. 
Dent. Rhodani, P. and R., is also a Dentalium. 
Dent. medium, Sow. (Trans. Geol. Soc., Lond., 2nd ser. IV, p. 343, pl. XV, fig. 4, accord- 
ing to Somerey, s own statement same as figured in Min. Conch., pl. 79) has the form of a true 
Dentalium, but one of the figures in the Transactions appears to exhibit a short slit near the posterior 
end, in which case the shell must be referred to Lutalis. 
Ce 
