380 CRETACEOUS GASTROPODA 
and others), the centrals and inner laterals being large and unequal, the outer 
laterals small, numerous and equally formed; the mantle is fissured in front, and 
there always are two equal, or sub-equal, branchial plumes present. 
Operculum generally wanting, rarely present, horny. 
Shell either spiral, conically elevated or depressed, with a number of perfora- 
tions or a slit in the outer margin of the aperture, or cupshaped with a peripherical 
slit, or with a lateral or apical perforation ; internally pearly or porcellanous. 
T include in this sub-order all the genera which have a slit or an opening in 
front of the mantle and two symmetrical, or very nearly equal, branchial plumes, 
thus essentially differing from the Scurrprancutata. The shell is extremely varia- 
ble in form, but with regard to the principal types, three families can be distin- 
guished — 
LV.—PLEUROTOMARIIDA. 
LVI.—HALIOTID A. 
LVIIL—FISS URELLIDA. 
These may again be separated into two divisions, one including the two first 
named families, possessing a spiral, internally generally nacreous shell, with a 
slit or a number of perforations in the outer lip, and the other only the last 
named one, having a cup-shaped, perforated or emarginated but not nacreous shell. 
To the two former the name SCHISMATOBRANCHIA was applied by Gray, while for 
the last one that of DICRANOBRANCHIA was introduced. I think, however, that the 
relations as existing between the two tribes warrant their classification in one 
sub-order to which I have applied the name FIssoBRANCHIATA, the separation of 
the gills into two more or less equilateral plumes being their characteristic distine- 
tion from the ScuTIBRANCHIATA; Gray’s two sub-orders will, therefore, be retained 
as tribes. 
I. Tribe,—Schismatobranchiata. 
Gray, Guide, 1857, p. 160. 
The animals have a distinct slit in the front part of the mantle, the two plumes 
are sub-equal, and placed on the left side of the gill cavity; the eyes are placed 
on thick bulgings on the outer side of the tentacles. 
Shell spiral, conically elevated, with a proportionate aperture, or depressed, 
with a very large one. 
LV. Family,—PLEUROTOMUARIID A. 
Of the very large number of genera belonging to this family the animal is 
only known of Scissurella (or Anatomus), it being closely allied to that of some 
Trocutp#. The foot is narrow, elongated, in front with two short lappets, posteriorly 
with two pairs of very long serrated cilia; tentacles two, very long and serrated, 
having the eyes at their base; mantle-slit in front distinct; gills consisting of two 
plumes; teeth—? The operculum is very thin, ovate (or circular ?) with an obscure, 
