PLEUROBRANCHUS. 191 



imens either living or alcoholic. Without these data, which are 

 supplied in very few except the European species, nothing can be 

 done toward this end. No monograph of the family has hitherto 

 been published. The shells of a few species have been figured in 

 the Conchologia Iconica ; and Vayssiere and Bergh have supplied 

 admirable anatomical details of the structure of the European 

 species. 



Synopsis of Genera andSubgenera. 



I. Mantle with the edge free and overhanging on all sides ; rhin- 

 ophores close together, inserted below anterior edge of mantle 

 on a frontal veil (Pleurobranchince). 

 a. Shell wholly immersed in the closed mantle. 



b. Penis or its foramen close to female orifice, 



PLEUROBRANCHUS, p. 191. 



bb. Penis or penial foramen some distance in front of 

 female orifice, mantle generally notched in front or 

 behind, OSCANIUS, p. 212. 



aa. Shell partly exposed by an orifice in the mantle, 



GYMNOTOPLAX, p. 210. 



II. Mantle passing without boundary into the broad anterior veil, 

 the rhinophores far apart, inserted on the surface of united 

 mantle and veil; no shell, (Pleurobranchceince}. 

 a. Right, left and posterior borders of mantle freely project- 

 ing, separated from the foot by a groove, KOONSIA, 



p. 221. 



aa. Posterior and left borders of mantle passing directly into 

 the integument of the foot, not free or projecting, 



PLEUROBRANCH^A, p. 223. 



b. Buccal rostrum when projected very large ; veil of 



moderate size Pleurobranchcea. 



bb. Buccal rostrum slender; body orbicular, the veil 



very large, crescentic Euselenops. 



III. Genus HALIOTINELLA (p. 209) is not included in the above 



synopsis, as its characters are unknown. The shell is similar 



to that of Pleurobranehus. 



Genus PLEUROBRANCHUS Cuvier, 1804. 



Pleurobranchus Cuv., Ann. du Mus. d'Hist. Nat., v, p. 275. 

 Berthella BLAINVILLE, Diet. Sci. Nat., xxxxi, p. 370, 1826, for 

 B.porosa=plumula. Cleantus LEACH, Synops. Moll. Gt. Brit., p. 



