MURICID^. 103 



The prosobranchiates may be divided into the following orders : 



Order Pectinibranchiata. 



The mollusks of this group have pectiniform branchiae ; that 

 is, composed of leaflets arranged Tike the teeth of a comb, in one 

 or two series or lines, and situated upon the upper wall of a 

 respirator}'' cavity formed b}^ the mantle, having an external 

 opening upon the side of the neck. Sexes separated, in different 

 individuals (dioecious). The shell is spiral. 



Order ScuTiBRANCHiAj^Ar"'''^ A^jt TTT A 



Branchiae pectinated, placed in.^a cavity in the upper partrof 

 the neck, or at the inferior edge of tile mantle, around the foot. 

 Dia?cious usually. Shell spiral ( globular "^r.,. 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 ; 

 branchiae partially or perfectly developed , forming a sort of nucleus 

 on the posterior part of the back — whence the ordinal name. 

 Dioecious. 



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. 



Order PECTINIBRANCHIATA. 



Family MURICID^. 



Shell spiral, turriculated, with an anterior canal ; the whorls 

 thickened b}^ varices or nodules at each rest-period in its growth. 

 Operculum with subapical or lateral and marginal nucleus. Lin- 

 gual dentition (x, 1*7, 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. 



