372 Mr. Kobert Garner's Malacological Notes. 



abdominal situation, these shells may be considered abdo- 

 minal rather than dorsal, and revolute or reversed as to the 

 direction of the spire. The siphon here is not situated dorsally 

 but ventrally; and the back of the head is directed towards the 

 spire — the reverse of what it is in Gastropods, Planorhis for 

 instance. But the shell or bone (so called) of the Sepia rather 

 answers to the dorsal plate of the pteropodous shell, as also 

 the fossil Beloteuthis ; and what is the expanded portion of the 

 Nautilus-shell is scarcely developed at all in the Sepia, though 

 more so in Belosepia. No doubt the light spongy laminated 

 part of the os sepice represents the hydrostatic cells of a Nau- 

 tilus and Ammonite and the phragmacone of the Belemnite 

 or of Spirulirostra ; the wavy lines seen by the lens on the 

 septa of the os sepice answer somewhat to the wavy sutures 

 of Ammonites, showing at least similarity of construction 

 histogenetically. In Spirulirostra we have a form connecting 

 Sepia with Spirula and Nautilus^ as Conoteuthis appears to 

 do the Belemnite with Loligo. Turrilites must have had an 

 unequal lateral development, like Gastropods*. 



In certain tectibranchiate mo Husks, as Gastropteron, Aply- 

 sia, Bullcea, Acei-as, and lanthina, we have a transition from 

 the Gastropods to the Pteropods, either the foot or mantle 

 being developed more or less into swimming-organs, whence 

 they might be arranged as natatory gastropods. The Ptero- 

 poda proper, though perhaps at present not at their zenith of 

 development, at least as regards size, appear in type to consti- 

 tute a characteristic class, and upon the whole a transition 

 between the two whose morphology has been dwelt upon, 

 though their likeness to the young veliferous Gastropod 

 may induce others to think differently. With Sejna, for 

 instance, Hyalma may be compared in respect of shell, also as 

 regards the abdominal situation of the branchiffi. Limacina 

 scarcely differs from Hyalcea except in having a spiral shell 

 and the pedal lamina a little more specialized. The tentacles 

 of Clio and Pneumodermon are apparently transitional in 

 structure to the arms or feelers of Cephalopods. Pteropoda 

 correspond with Gastropods as to the dextral position of the 

 rectum ; but the heart is also dextral or rather abdominal, and 

 they vary somewhat as to the openings of their androgynous 

 generative organsf. Four of the six cephalic ganglia lie in 



* The Ammonite appears to have been furnished with an operculum. 

 The Argonaut-shell has been compared to the ovigerous float of lan- 

 thina : perhaps it has not been conclusively shown by Madame Power 

 what part mainly secretes it, the veliferous arms or the mantle ; if the 

 former, we might consider it to answer to a developed operculum. 



t The above we believe to be correct ; but there is some confusion in 

 descriptions. 



