NAUTILOIDEA 395 



spiral, last whorl straight, containing- tlie l)ody-chaniher, often 

 greatly prolonged. Principal genera : Lif/aitcs, Oiihidioceras. — 

 Ordovician and Silurian. 



Fa^l 9. Tri)c](oceratidac. — Shell .helicoid, witli 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 oi- less 

 overlapping, septa simple, siphuncle central 

 or sub-central, aperture not contracted. /""^^^ 



The 'tentacles' are about 90 in numlier, v aI J 



and consist of four groups each of 12 or 13 

 labial tentacles surrounding the mouth, two '^^ /^HIn. 



groups each of 17 larger (brachial) tentacles / \ 



on each side of the head, two thicker tenta- . / _ _ j\ 



cles which combine to form the ' hood,' and p^c~v-}~~~-| 



two small tentacles on each side of the eve. *'V^-^^4^^ 

 When the animal swims, the tentacles are x ^^ ^ -yb^ -d , 



extended radially from the head, somewhat V^-/ 



like those of a sea-anemone. The direction of '-"-^ 



the many i)airs of tentacles at constant l)ut ^'''- ^^^',7r' ^"Tf!'"' 



•'J- cerrts eUipticuniM Coy, 



different angles from the head is the most Silurian : B, aperture 

 striking feature in the living muHlus, and '^l^af st^^Lon of 

 accounts for its being described, when seen sipiumcie. (After 

 on the surface, as ' a shell with soniethin<; like 



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

 l)ut is formed Ijy 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 TrocJiolites, Gyroceras, Hercoceras, Discites, 

 Attiria. — 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, Barfn'trs.- — Silurian and Devonian. 



(h) J^ntH'ipJionata. — Septal necks tlirected/ov7/Y«W.s\ 

 ' Saville Kent, Proc. Jloij. Soc. Queensland, vi. p. 229. 



