MURICIDA. 103 
The prosobranchiates may be divided into the following orders: 
Order PECTINIBRANCHIATA. 
The mollusks of this group have pectiniform branchiz ; that 
is, composed of leaflets arranged like the teeth of a comb, in one 
or two series or lines, and situated upon the upper wall of a 
respiratory cavity formed by the mantle, having an external 
opening upon the side of the neck. Sexes separated, in different 
individuals (dicecious). The shell is spiral. 
Order ScuTTBRANCHIATA. 
Branchiez pectinated, placed in a cavity in the upper part of 
the neck, or at the inferior edge of the mantle, around the foot. 
Dicecious usually. Shell spiral (globular or pyramidal) or 
conical, holostomate, or with entire margins without anterior 
canal. 
Order POLYPLACOPHORA. 
Shell multivalve, consisting of eight separate pieces inserted 
upon the back of the animal and surrounded by a mantle-border. 
Order NUCLEOBRANCHIATA. 
Pelagic animals, swimming by means of fin-like lobes of the 
foot ; with or without a shell, the latter being glassy, transparent ; 
branchie partially or perfectly developed, forming a sort of nucleus 
on the posterior part of the back—whence the ordinal name. 
Dicecious. 
This small group appears to form a connection with the Nudi- 
branchiate division of the Opisthobranchiata on the one hand, 
whilst on the other its specialized swimming organs and the 
consistency of its shell ally it to the Pteropoda. 
OrpeR PECTINIBRANCHIATA. 
Famity MURICID &. 
Shell spiral, turriculated, with an anterior canal; the whorls 
thickened by varices or nodules at each rest-period in its growth. 
Operculum with subapical or lateral and marginal nucleus. Lin- 
gual dentition (x, 17, 19). 
Subfamily muricin#. Three or more varices on each whorl, 
the varices being nodulous, foliated or spinose; canal long or 
short, but well-marked. 
Subfamily purpurin®. Without varices, but tuberculate ; 
columella flattened or patulous ; basal canal very short or a mere 
notch. 
