REPORT ON THE PTEROPODA. 11 



more tough than those of some Pneumonoderma. Moreover, the Gymnosomata form a 

 group of too low a status, and contain forms differing too little from one another to 

 enable one to establish within it more important divisions than families. 



Three of the seven genera I have admitted — Pneumonoderma, Spongiobranch&a, 

 and Dexiobranchsea — show a very close resemblance to one another, and clearly 

 differ from the other genera by the presence of acetabuhferous buccal appendages and 

 of a right lateral gill ; they constitute the family Pneumonoderrnatidae. 



Clione and Halopsyche are quite separated from the other genera by the complete 

 want of branchial apparatus ; they differ as much by the form of the body as by that of 

 the fins, by the buccal appendages, &c, and respectively represent the families of the 

 Clionidae and Halopsychidae. 



Clionopsis, till now ranked near Clione, because its organisation was imperfectly 

 known, cannot remain among the Clionidae. In several characters (the presence of a 

 tetraradiate posterior gill and of a dorsal spot) this genus resembles some Pneumono- 

 dermatidae more than Clione; but other more important characters (the absence of a 

 lateral gill and of acetabuhferous buccal appendages, the presence of a retractile 

 proboscis of extraordinary length) exclude it from so natural and so well-characterised 

 a group, which includes Pneumonoderma, Spongiobranchsea, and Dexiobranchsea. There- 

 fore Clionopsis represents a special family — Clionopsidae. 



As to the new genus Notobranchsea, it must be excluded from all the above cited 

 families : from the Clionidae and Halopsychidag, by the presence of a gill ; from the 

 Pneumonoderrnatidae, by the want of the lateral gill and suckers ; and from the Clionop- 

 sidae, by the presence of buccal appendages and by the form of the gill and the foot. 



A new family must therefore be established, Notobranchasidae, for this new genus. 



Hence, the Gymnosomata are divisible into five families, of which the first 

 (Pneumonodermatidae) includes three genera. 



Family I. Pneumonodermatidae. 



1840. Pneumodermidx, Gray (pars), Synopsis of the Contents of the British Museum, p. 86. 



1842. Pneumodermidx, d'Orbigny, Paleontologie franeaise, terrains cr(3taces, t. ii. p. 4. 



1846. Pneumodermoidx, Agassiz, Nomenclator zoologicus, Index, p. 299. 



1852. Clios, Souleyet (piars), Histoire naturelle des Mollusqnes Pteropodes, p. 74. 



1855. Clioidea, Gegenbaur {pan), Uiitersuchungen iiber Pteropoden und Heteropoden, p. 212. 



1858. Pneinnodermonidx, H. and A. Adams (pars), The Genera of recent Mollusca, vol. L p. 62. 



1862. Pneumodermidx, Bronn, Die Klassen und Ordnungen des Thierreichs, Bd. iii. p. 645.- 



1871. Pneumodermatidx, Dall, Description of Sixty New Forms of Molluscs from the West Coast 



of North America, Amer. Journ. of Couchology, vol. vi. p. 139. 



1881. Cliidx, Fischer (pars), Manuel de Conchyliologie, p. 243. 



Characters. — Suckers on the ventral side of the protrusible anterior portion of the 

 buccal cavity. A lateral gill on the right side. A jaw. Pigmented skin. 



