XIII 



NAUTILOIDEA 



395 



spiral, last whorl straight, containing the body-chamber, often 

 greatly prolonged. Principal genera : Lituites^ Ophidioeeras. — 

 Ordovician and Silurian. 



Fam. 9. Troclioceratidae. — Shell helicoid, with seldom more 

 than two whorls, dextral or sinistral, last whorl sometimes 

 partly uncoiled. Principal genera: Trochoceras, Adelphoceras. 

 — Ordovician to Devonian. 



Fam. 10. Nautilidae. — Shell with few whorls, more or less 

 overlapping, septa simple, siphuncle central 

 or sub-central, aperture not contracted. 



The ' tentacles ' are about 90 in number, 

 and consist of four groups each of 12 or 13 

 labial tentacles surrounding the mouth, two 

 groups each of 17 larger (brachial) tentacles 

 on each side of the head, two thicker tenta- 

 cles which combine to form the 'hood,' and 

 two small tentacles on each side of the eye. 

 When the animal swims, the tentacles are 

 extended radially from the head, somewhat 

 like those of a sea-anemone. The direction 

 of the many pairs of tentacles at constant but 

 different angles from the head, is the most 

 striking feature in the living Nautilus^ and 

 accounts for its being described, when seen 

 on the surface, as ' a shell with something like 

 a cauliflower sticking out of it.' ^ The funnel is not a complete tube, 

 but is formed by the overlapping of the margins of two thin fleshy 

 lobes (which are probably morphologically epipodia), so that when 

 the two lobes are parted, a broad canal appears, leading to the bran- 

 chial cavity. The head is conical, and the mouth and its appendages 

 can be retracted into a sort of sheath, over which fits the 'hood.' 



Other genera are TrocJiolltes, G-yroceras^ Hercoceras^ Dlscites, 

 Aturia. — Ordovician to present time. 



Fam. 11. Bactritidae. — Shell straight, conical, siphuncle 

 small, marginal, septal necks long, funnel-shaped, sutures undu- 

 lating, with a sinus corresponding to the siphuncle. This family, 

 from the form of its sutures, appears to constitute a passage to the 

 Ammonoidea. Single genus, Bactrites. — Silurian and Devonian. 



(h) Prosiphonata. — Septal necks directed /or^^arc^s. 



The two genera are BatJunoceras (Ordovician), shell straight, 

 1 Saville Kent, Proc. Boy. Soc. Queensland, vi. p. 229. 



Fig. 255. — A, Gompho- 

 ceras eUipticum 



M'Coy, Silurian: B, 

 aperture (ap) of 

 same ; s, s, septa ; si, 

 position of siphuncle. 

 (After Blake.) 



