BLAINVILLE. 545 



margin of the cloak ; body naked, oval, more or less 

 tubercular. 

 Gen. : Phyllidia, Linguella. 



Order V. Nucleobranchiata. Organs of respiration in 

 the form of symmetrical filaments grouped with the 

 digestive organs in a nucleus situated on the superior 

 and, generally, the posterior part of the back ; the skin 

 thick, naked, and mucous. Shell symmetrical, more or 

 less spiral longitudinally or from behind forwards, and 

 very thin. 



Family 1. Nectopoda. A ventral foot compressed 

 into a rounded swimmer. Gen. : Pterotrachea, 

 Carinaria. 

 ,, 2. Pteropoda. A winglike appendage on 

 each side of the body for swimming. Gen. : 

 Atlanta, Spiratella, Argonauta. 



Subclass III. P. HERMAPHRODITA. 



Every individual of the species alike and unisexual. Shell 

 simple, covering, rarely a little spiral, symmetrical or not, 

 non-operculate. 



Section I. Organs of ■respiration and shell symmetrical. 

 Order I. Cirrhobranchiata. Organs of respiration fila- 

 mentous, seated on two sessile lobes above the neck. 

 Gen. : Dentalium. 

 Order II. Cervicobranchiata. Organs of respiration 

 in a large cavity above the' neck, with a wide opening 

 in front. 



Family I. Retifera. Organs of respiration retiform 

 on the floor of the branchial cavity. Gen. : Pa- 

 tella. 

 ,, 2. Branchifera. Organs of respiration two 

 large equal combs. Gen. : Fissurella, Emargi- 

 nula, Parmaphorus. 

 Section II. Organs of respiration and shell non-symmetrical. 

 Order III. Scutibranchiata. Organs of respiration 

 aqueous, and covered by a subspiral or patelliform shell. 

 Family 1. Otidea. Respiratory organs on the left 

 side. Gen. : Haliotis, Ancylus. 

 ,, 2. Calyptracea. Branchiae above the origin 

 of the back. Ex. Crepidula, Calyptraea, Capulus, 

 Hipponyx. 



Class III. ACEPHALOPHORA = Acephales, Cuv. 

 Order I. PALLiOBRANCHiATA=Brachiopodcs, Cuv. 



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