170 
THE LIAS AMMONITES. 
Order L— DIBRANCHIATA, Owen, 1832. 
AcETABULiFERA, (T OrUgvy, 1840. 
Body naked, built for swimming. Head distinct, eyes large, sessile, prominent. 
Mandibles horny, moving in a vertical direction, arms eight to ten, provided with 
acetabula. Branchiae two. Heart with three chambers, one systemic and two branchial. 
Ink-bag always present. Eunnel parietes entire. Shell in one family monothalamous 
external, and not adherent to the body of the animal. Other families have an internal 
siphoniferous polythalamous shell, or the rudiments of an internal skeleton in various 
stages of development are found in the substance of the mantle. 
With the exception of the Nautilus all the living Cephalopods belong to this order ; 
they appeared, as far as we at present know, for the first time in the Trias. One family 
only, the Belemnitidce, is entirely extinct. 
Section a. — Octopoda, Leach, 1817. 
Arms eight, suckers sessile. Eyes fixed, inca- 
pable of rotation. Nuchal band between the head 
and mantle broad. Eunnel without a valve. No 
distinct nidamental gland. Shell external, one-celled, 
and non-adherent. Internal shell, when present, con- 
sisting of rudimentary pieces. 
1. Family Octopodid^, Gray, 1847. — Arms 
similar, elongated, and united at their base by a 
membrane ; the eight arms are each provided with a 
double series of sessile acetabula, without bony circles. 
Shell represented by two short styles, according to 
Owen, embedded in the substance of the mantle. 
The Octopus vulgaris, Lam, (fig. 14), is the type of this family. 
2. Argon AUTiD^, ^eeye, 1841. — The two dorsal arms of the female Argonaut 
Fig. 14. — Octopus vulgaris, Lamarck. 
Fig. 15.- — Argonauta aigo, Linn. 
Fig. 16. — Argonauta argo. Linn. 
are expanded at their extremities, and secrete a symmetrical involuted shell, which is thin 
and translucent, and not moulded on the body of the animal, nor attached to it either by 
muscles or a siphonal tube. The coil of the spire is occupied by clusters of ova (fig. 16). 
