REPORT ON THE PTEROPODA. 23 



regard to the posterior gill, de Blainville ' gives it as only formed of three sides (the right 

 side of the quadrangular crest wanting). Several writers have figured fringes on the 

 four sides of this crest; in this respect the figure of Rang 2 is much more exact, but the 

 one given by d'Orbigny, 3 and republished by so many modern authors, 4 is quite erroneous, 

 and makes this gill resemble fins. As for the lateral gill, it is generally forgotten or 

 figured in a shapeless manner. Lastly, in regard to the structure of these gills, there is 

 no figure, not even those' of Souleyet, 6 which are by far the best and most exact, that 

 shows the true conformation of the fringes. 



These fringes are differently formed in the different species. Those of Pneumonoderma 

 souleyeti are very little developed, and only form simple undulations at the base of the 

 crests. Those of Pneumonoderma pacificum form well-marked and regular undulations, 

 those of one side alternating with those of the other, and resembling the fringes of the 

 gill of Clionop>sis grandis. In Pneumonoderma boasi there are a few fringes more 

 distinct, shrunk at their base, but markedly separate. Lastly, in the group including 

 Pueumonoderma violaccum, Pneumonoderma peroni, and Pneumonoderma mediter- 

 raneum, the fringes of the gills show their greatest specialisation; they are numerous, 

 nearly contiguous, and subdivided into parallel ramifications, very close to one another, 

 directed from the base of the fringe to its distal extremity (fig. 3, 3). The conformation 

 of the gills is very useful for the distinction of the species. 



Huxley, 6 and several others after him, as Woodward 7 and Fischer, 8 have said that 

 Pneumonoderma possesses a "small anomalous shell." I have examined a great many 

 specimens of Pneumonoderma of all sizes, and even old larvae, and I can assert that 

 like all the other Gymnosomata, Pneumonoderma is absolutely devoid of mantle skin 

 and shell. It is quite probable on the other hand, whatever Lankester may say, 9 that 

 the larvae of Pneumonoderma bear a larval shell in the beginning of their development, 

 as in Clione. 10 



The discrimination of the species of Pneumonoderma is certainly the most laborious 

 part of the systematic treatment of the Gymnosomata. A considerable number of species 

 have been described, but for the most part the descriptions are imperfect and insufficient, 

 and the type specimens have been lost. 



A comparative and synthetical study of all the specific forms of this genus has not 

 yet been made, and with the materials now available, it might not even yet give 



1 Manuel de Malacologie et de Conchyliologie, pi. xliii. figs. 4, 46. 



2 Histoire Naturelle des Mollusques Pteropodes, pi. ix. fig. 5. 



3 Voyage dans l'Ame'rique me>idionale, t. v., pi. ix. fig. 11. 



4 Bronn's Thierreieh, Claus' Text-book, Lankester's Mollusca, in Encyclopaedia Britannica. 



5 Voyage de la Bonite, Zoologie, pi. xv. figs. 40-42. 



On the Morphology of the Cephalous Mollusca, Phil. Trans., 1853, p. 40. 



7 A Manual of the Mollusca, p. 208. 



8 Manuel de Conchyliologie, p. 423. 



9 Mollusca; Encyclopaedia Britannica, ed. 9, vol. xvi. p. 666. 



10 Fol, Sur le developpement des Pteropodes, Archives d. Zool. exper., sir. 1, t. iv., pi. x. figs. 6, 7. 



