390 CRETACEOUS GASTROPODA 
but both of them have depressed whorls with the spinal tubes placed at the peri- 
phery, not on the upper surface. 
Il. Tribe. —Dicranobranchiata. 
Gray, Guide, 1857, p. 162. 
The animals belonging to this division, introduced by Gray for the symmetrical 
forms of the FissoBRANCHIATA, have two equal branchial plumes, placed on the 
back of the neck, and a cup-shaped shell, which only in very young specimens 
occasionally has a sub-spiral apex. All the genera are united in only one family. 
LVII. Family, —FISSU RELLIDA. 
H. and A. Adams, Gen. I, p. 444; Gray, Guide, 1857, p. 162. 
Animal with a thick oval foot, short, broad muzzle, sub-cylindrical tentacles 
with the eyes sessile on small bulgings at their outer base; the mantle is 
fissured or perforated in front; there is usually a fringe, composed of a small 
number of cirri or short filaments, present on the upper part of the foot; the head 
lobes are sometimes rudimentary, but usually not developed at all; the central 
tooth is simply incurved, more or less broad, the inner laterals dissimilar, the three 
or four pairs next to the central one being small, hook-like; one on each side is very 
large and denticulated, and then follow the numerous outer laterals which are 
slender, similarly formed, having the tips pointed and incurved. Dr. Williams (Ann. 
mag. nat, hist., 1865, XVI, p. 419) sometime ago stated that the branchial plumes 
of Emarginula each have only a single row of strands or fillets, while those of 
Fissurella have a double row, one on each side of a central axis. Iam not aware 
whether these observations have been extended to other allied genera; but if these 
distinctions of the branchial leaves can be traced in other forms, it is just possible 
that the genera with a slit in the apertural margin can he separated into a distinct 
family or sub-family from those which have the shell pierced. 
Shell cup-shaped, in young specimens sub-spiral at the apex, not pearly within, 
margin of the aperture with a slit or a slight emargination, generally in front, rarely 
somewhat on one side, or with a perforation at or near the apex ; operculum none. 
The Fissvrettips# are mostly littoral and phytophagous; they are often found 
on coral reefs and sparingly on the coasts of all countries; the largest number 
of species are known frcm the Indian Archipelago and from South America. 
Fossil species are not very rare in the tertiaries, but in the mesozoic deposits they 
rapidly decrease in number, being chiefly represented by the genera Limarginula 
and Rimula, while those of the palzeozoic formations are very scarce and rather 
peculiar, so as to form probably a separate group. 
The following will give a short review of the generic characters of the shells 
belonging to this family; a monograph of it has been published a few years back 
in Sowerby’s Thesaurus Conchyliorum, but I am sorry that I cannot refer to the 
work just at present, it not having been yet received in Calcutta. 
