SIPHONOFODA.] MOLLUSCA 669 Family 1. Xautilidie. Genera : [Orthoccras], Breyn. ; [Cyrtoccras], Goldfuss ; [Gfompho- ceras], Minister ; [Phragmoccras], Brod. ; [Gyroccras], Meyer ; [Ascoccras], Barraude ; [Oncoccras], Hall; [Lituites], Breyn.; [Trochoceras], Barraude; Nautilus, L. (figs. 88, 89, 90, &c.) ; [Clymenia], Miinst. ; [Nothoccras], Barraude. Family 2. Ammonitidx. Genera: [Bactritcs], Sanderg. ; [Goniatitcs], de Haan ; [Rhabdo- ceras], Hauer ; [C lydonitefi], Haucr ; [Cochloccras], Hauer ; [Baculina], d Orb. ; [Ceratitcs], de Haan ; [Baculitcs], Lam. ; [Toawcmw], d Orb.; [Crioccras], Leveille ; [PlycJwceras], d Orb. ; [Hamites], Parkinson; [Ancyloccras], d Orb.; [Scaphites], Parkinson; [Ammonites], Breyn.; [Turrilitcs], Lam. ; [Hdio- ccras], d Orb. ; [Hctcroccms], d Orb. N.B. The names in brackets are those of extinct genera. Order 2. Dibranchiata ( = Holosiphona, Acetabulifera). Characters. Siphonopodous Cephalopoda in which the inflected lateral margins of the mid-foot are fused so as to form a complete tubular siphon (fig. 90, i). The circum- oral lobes of the fore-foot carry suckers disposed upon them in rows (as in the Pteropod Pneumodermon), not tentacles (see figs. 92, 95, 96). There is a single pair of typical ctenidia (fig. 103) acting as gills (hence Dibranchiata), and Fio. 92. Sepia offlcinalis, L., half the natural size, as seen when dead, the long prehensile arms being withdrawn from the pouches at the side of the head, in which they are carried during life when not actually in use. a, neck ; 6, lateral fin of the mantle-sac ; c, the eight shorter arms of the fore-foot ; d, the two long prehensile arms ; e, the eyes. a single pair of nephridia opening by apertures right and left of the median anus (fig. 103, ?), and by similar internal pores into the pericardial chamber, which consequently does not open directly to the surface as in Nautilus. The ovi ducts are sometimes paired right and left (Octopoda), sometimes that of one side only is developed (Decapoda, except Ommastrephes). The sperm-duct is always single except, according to Keferstein, in Eledone moschata. A plate-like shell is developed in a closed sac formed by the mantle (figs. 98, 99), except in the Octopoda, which have none, and in Spirula (fig. 100, D) and the extinct Belemni- tidie, which have a small chambered shell resembling that of Nautilus with or without the addition of plate-like and cylindrical accessory developments (fig. 100, C). The pair of cephalic eyes are highly-developed vesicles with a refractive lens (fig. 120), cornea, and lid-folds, the vesicle being in the embryo an open sac like that of Nautilus (fig. 119). Osphradia are not present, but cephalic olfac tory organs are recognized. One or two pairs of large salivary glands with long ducts are present. An ink-sac formed as a diverticulum of the rectum and opening near the anus is present in all Dibranchiata (fig. 103, t), and has been detected even in the fossil Belemnitidae. Branchial hearts are developed on the two branchial advehent blood vessels (fig. 104, vc m). The Dibranchiata are divisible into two sub-orders, accord ing to the number and character of the arm-like sucker- bearing processes of the fore-foot. (from the Atlantic Ocean). Sub-order 1. Decapoda. Characters. Dibranchiata with the fore-foot drawn out into eight shorter and two longer arms (prehensile arms), the latter being placed right and left between the third and fourth shorter arms. The suckers are stalked and strengthened by a horny ring. The eyes are large and have a horizontal in place of a sphincter-like lid. The body is elongated and provided with lateral iins (lamelliforin expansions of the mantle). The mouth has a buccal membrane. The mantle-margin is locked to the base of the siphon by a specially- developed cartilaginous apparatus. Numerous water-pores are pre sent in the head and anterior region of the body, leading into re cesses of the integument of unknown significance. The oviduct is single ; large nidamental glands are present. The viscero-pericar- dial space is large, and lodges the ovary (Sepia). There is always a shell present which is enclosed by the upgrowth of the mantle, so as to become " internal." Section a. Decapoda Calciphora. Character. Internal shell calcareous. Family 1. Spirulidse. Genus : Spirula, Lam. (fig. 100, D). Family 2. Belemnitidae. Genera: [Spvruliroatra], d Orb. (fig. 100, C) ; [Beloptera], Dcsh. ; [Belemnosis], Edw. ; [Conotcuthis], d Orb. (fig. 100, A) ; [Acan- thotcuthis], R. Wag.; [Bekmnites], Lister, 1678; [Uelemnitella], d Orb. ; [Xiphoteuthis], Huxley. Family 3. Scpwdx. Genera: Sepia, L. (figs. 92, 98, &c.); [Belosepia], Voltz ; Cocco-
tenth is, Owen.