192 PLEUROBRAKCUUS. 



28, 1 852, for C. montagui=plumula. Cleanthus Leach, 1819, GRAY, 

 P. Z. S., 1847, p. 163.? Westernia &nd Gervisia Quoy & Gaim., 

 Ms. according to de Blainville, Man. de Malac., Addit. et Correct., 

 p. 654, 1827. t Discoides RENIERI, (?Tav. Class., 1807) according 

 to Agassiz, Nomencl. Zool. 



Body oval, the mantle about the size of the foot, free and pro- 

 jecting at the edges all around ; the rhinophores contiguous, in- 

 serted below it, above an expansion of the integument or " veil." 

 Gill bipinnate ; male and female generative orifices contiguous or 

 almost united. Shell present, auriculate. Type P- peronii, 

 Cuvier. 



In this genus, which comprises a majority of the species of the 

 family, the mantle is smooth or at least not conspicuously tubercular, 

 the rhinophores are inserted below the front edge of the mantle, and 

 the genital orifices are closer together than in other Tectibranehiata. 

 The internal classification of the group is still unsettled, awaiting 

 more light ; but the following arrangement may be suggested : 

 I. Shell completely buried in the closed mantle. 



a. Male and female genital orifices contiguous but distinct, 



Section Pleurobranchus. 



aa. Male and female orifices united in a single elongated 

 papilla or pit Section Berthella. 



It is at present impossible to make any more logical arrangement 

 than a geographic one of the species of this group. So many spe- 

 cies are still known only in the most superficial way, that there is no 

 basis upon which to build a synoptical table or " key " for their de- 

 termination. It is known that all European species belong to Ber- 

 thella; most Indo-Pacific forms upon which data are extant, belong 

 to the restricted group Pleurobranchus. 



Geographic distribution of species. 



European Seas : P. plumula, Atlantic. 



P. stellatus, aurantiacus, monterosatoi, perforatus, 



brevifrons, Mediterranean. 



American : P. areolatus, quadridens, circularis, West Indies. 

 P. patagonicus, S.-E. coast Argentina. 

 P. digueti, Lower California. 



Polynesian : P. delicatus, ovalis,pellucidus,tessellatus, marginatus, 

 rufus, varians. 



