67 



these forms I shall have to discuss further on, as they were obtained within the area of 

 exploration of the Siboga. 



The material of this expedition contains three species, two of which are new. ( )ne 

 species, recorded from the East-Indian Archipelago has not been obtained. 



I may lay stress on the fact that it is often very difficult to distinguish the species 

 from each other. The body being strongly contracted, it is by no means easy to discover the 

 exact shape of the lateral and the posterior gill; and the Iniccal appendages can be examined 

 generally only by dissection of the head:cavity, in which they are retracted. Added to this 

 the excessive smallness of some species (not yet 2 mm.), one can understand the difficulty 

 with which the systematic classification of Pnctiiiionodcrnia is connected. 



I. Pneiiuwnoderiiia pcroni (Lamarck). 



1815. Aegle ciicnllata Oken, Lehrbuch der Zoologie, Bd. I, p. 327. 



1 8 19. Ptieumodernion pcroni Lamaick, Histoire natuicllc des Animaux sans Vertebres, vol. 



VI, p. 294. 

 1887. Pneiimoderma pcroni Fischer, Manuel de Conchyliologie, p. 423. 

 1887. Pnciiinonodcniia pcroni Pclseneer, Challenger Rep. LVIII, p. 29, pi. II, fig. 2. 



Living animal: 



Stat. 109. Pulu Tongkil, Sulu Archipelago, i spec. 



Only one specimen was caught during the whole cruise of the expedition. It is very 

 contracted; the skin is plicated in large folds and of a greyish colour. In the shape of the 

 gills, and in the disposition of the acetabuliferous appendages, the specimen entirely agrees 

 with the descriptions of Boas and Pelseneer. The length is only y mm. 



*2. PnciimoiiodcrDia incdite7'ranejim (van Beneden). 



1838. Pncumodcrinon mediterrancnm van Beneden, Note sur une nouvelle espece de Pneumo- 



derme, Mem. Acad. Sci. Brux., vol. XI, p. 13, pi. Ill, figs, i — 2. 

 1852. Pneuuiodcrinon peronii (part.) Souleyet, Voyage de la Bonite, vol. II, p. 274, pi. XIV, 



figs. 13, 14, 16. 

 1852. } Pncumodcrinon peronii (part.) Rang, Histoire naturelle des Mollusques Pteropodes, 



pi. XI, figs. 14 — 19. 

 1855. Pncuniodcrmon niediterraneum Gegenbaur, Untersuch. iiber Pterop. u. Heterop. p. 213. 

 1859. Spongiobranchea australis (part.) Chenu, Manuel de Conchyliologie, vol. I, p. 116, fig. 509. 

 1873. Pncumodcrinon peronii Costa, Pteropodi della Fauna di Napoli, p. 22, pi. V, fig. i — 3. 

 1886. Pncumodcrinon macrocotyliiin Boas, Spolia atlantica; Bidrag til Pteropodernes Morfologi, 



etc., K. dansk. Vidensk. Selsk. Skr., 6 Raekke, Bd. IV, p. 152. 



1886. Pncumodcrmon andebardi Locard, Prodrome de la Malacologie frangaise, Mollusques 



marins, p. 19. 



1887. Fncuinodcnna mcditcrraneum Fischer, Manuel de Conchyliologie, p. 423. 



1887. Pneumonodcrma niediterraneum Pelseneer, Chall. Rep. LVIII, p. 26, pi. I, fig. 8. pi. II, fig. i. 



According to Boas, this species has been caught in the Indian Ocean (5'^24'S., 87°56'E. 

 and 10° S., i04°E.) and in the China Sea (16° X., ii5°2o'E.). 



