CAHBONIPEROUS AGE. PERMIAN PERIOD. 63 



CEPHALOPODA. 



Order Tetrabrancliiata. 



Family NAUTILID^. 



Shell curved, involute, or rarely spiral ; outer or last chamber capacious, 

 sometimes deflected from the curve of the inner whorls, and more or less 

 straightened ; aperture usually sinuous on the dorsal or outer side. Septa 

 simple, or with a few undivided lateral lobes or flexures ; concave on the 

 side facing the aperture. Siphon varying in its position between the 

 inner and outer, or dorsal and ventral margins, according to the genera 

 and species ; rarely (in older extinct groups) occupied by an internal 

 organic deposit ; nearly always piercing the septa backwards from the 

 aperture ; envelope usually solid and persistent. 



For what is known in regard to the structure of the anim&l in the recent typical 

 genus of this family, we are mainly indebted to Prof. Richard Owen, of London. 

 According to this distinguished comparative anatomist, the powerful parrot-like 

 mandibles of the recent Nautilus pompilus are surrounded by a fleshy lip, around 

 which are four groups of labial tentacles, numbering twelve to thirteen each. Out- 

 side of these, on each side of the head, are thirty-six brachial tentacles or arms, 

 arranged in a double series ; the dorsal pair being expanded and connected so as 

 to form a hood, which partly closes the aperture of the shell when the other 

 appendages are retracted. The tentacles are laminated on their inner sides, and 

 capable of being drawn within sheaths apparently homologous with the eight arms 

 of the Cuttle-fish. There are also four ocular tentacles — one behind and one 

 before each eye. The respiratory funnel is formed by the folding of a thick lobe 

 Avhich extends laterally on each side of the head, with the free edge directed 

 backwards into the branchial cavity. The mantle is firm and muscular as far back 

 as the line of the shell muscles, beyond which it is transparent. Its margin is 

 entire, and extends to the edge of the shell. The siphon is vascular, and connected 

 with the pericardium. 



Most Palaeontologists include in the family Nautilidce, an extensive group of older 

 fossil shells presenting a great diversity of forms and other characters : such, for 

 instance, as Phragmoceras, Oomphoceras, Endoceras, Orthoceras, Euronia, Aplo- 

 ceras, &c. — probably belonging to one or more distinct families. As here defined, 

 the family Nautilidce is intended to include the following groups: Nautilus, 



