DESCENT OF THE OCTOPODA. 91 



forms with lateral ears to the mouth, while in other characters they 

 are similar, suture line, ornaments, &c. ; hence they conclude that 

 the large ones are the females, and the small, males. But since the 

 lobings of the Septa must be determined by the position of soft 

 parts, and it is impossible that the female with her complex sexual 

 apparatus should produce the same septa as a male with his com- 

 parative simple one, it seems inevitable that we must call these 

 forms different genera, as Zittel does,( 19 J such as OeJcotraustes and 

 Oppelia. 



If this view be true that Argonauta is closely allied to the 

 Ammonites, (and it has been gaining sensibly in favour in the last 

 few years, for we find the Ammonites set down as Dibranchs in the 

 " Zoological Record,") then Owen's group Dibranchiata includes a 

 direct " mutation " (to use Prof. Blake's term)(* 20 ) of the Ammonites, 

 and highly specialized " divergents " of the Nautili (Belemnites, &c.) ; 

 so that the division has now become artificial and strained. And not 

 for this reason alone. 



In the Dibranchiata, the skin is richly supplied with blood 

 vessels, and a considerable amount of cutaneous respiration goes on, 

 so that the least possible number and size of gills is required to keep 

 the blood in its normal condition ; but in animals enclosed on all 

 sides with a shell, a greater gill-surface will be required. This may 

 be obtained cither by increasing the number or size of the gills. 

 Since the only living Nautilus has done the former, it is probable 

 that the Ammonites did the same, and, possibly to a larger extent. 

 In Arcestes, for instance, we get a body chamber over one whorl in 

 length, and in some cases approaching two, the aperture is very 

 narrow, and the chamber low and flat ; here there would not have 

 been room for two large gills of the requisite size, or even four, but 

 there might easily have been a row of several pairs of small branchiae. 

 Therefore there seems as much reason to class the Ammonites and 

 Nautili with long chambers as Multibranchiata, as the conventional 

 Tetrabranchiata. 



However, the number of gills must always remain hypothetical 

 in these extinct forms, and a new division with new names seems 

 desirable, FoordO 21 ) and Blake ( 22 ) have taken Mr. Bather to divide 

 the Cephalopoda into the Ammonoidea, Nautiloidea, and Coleoidea, 

 though he himself seems to restrict his new name to the Belemnites. 

 However the two first groups are bound together by so many bonds 

 of similarity, that it seems unwise to separate them, especially as 



(19). Ilandbucli der Palceontologie, Bd. II. 



(20). Presidential Address, Geol. Assoc, 1893. 



(21). Brit. Mus. Cat. Cephalop., Vol. I, 1888, p. 10. 



(22). Ann. Mag. Nat. Hist., 1888, p. 377. 



