132 



MOLLUSCA 



Family 1. Nautilidse. 

 Genera : [Orthoceras], Breyn. ; [Cyrtoceras], Goldfuss ; [Gompho- 



ceras], Munster ; [Phragmoceras], Brod. ; [Gijroccras], Meyer ; 



[Ascoceras], Barraude ; [Oncoccms], Hall ; [Lituitcs], Breyn. ; 



[Trochoceras], Barraude; Nautilus, L. (figs. 88, 89, 90, &c.) ; 



[Cfymenia], Miiiist. ; [Nothoceras], Barraude. 

 Family 2. Ammonitidee. 



Genera : [Bactrites], Sanderg. ; [Goniatitcs], de Haan ; [Khabdo- 



ceras], Hauer ; [Clydonitcs], Hauer ; [CoMoceras], Hauer ; 



[Baculina], d'Orb. ; [Ceratites], de Haan ; [Baculites], Lam. ; 



[Toxoceras], d'Orb.; [Criocems], Leveille ; [Ptychoceras],&'0rb. ; 



[Hamites], Parkinson ; [Ancyloceras], d'Orb. ; [Scaphites], 



Parkinson ; [Ammonites], Breyn.; [ Tu rrilites], Lam. ; [Helio- 



ceras], d'Orb.; [Heteroceras], 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. 96, 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 offidnalis, 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 prehensile arms being withdrawn from the pouches at the side of the head, 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 anus 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, r), 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- 

 tidse, 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. 1 20), 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, i), and has 

 been detected even in the fossil Belemnitidae. Branchial 

 hearts are developed on the two branchial advehent blood- 

 vessels (fig. 104, vc', vf). 



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. 



FIG. 93. Decapodous Siphonopods; one-fourth the natural size linear. A. 

 Cheiroteuthis Veranyi, d'Orb. (from the Mediterranean). B. Thysanoteuthis 

 rhombus, Troschel (from Messina). C. Loligopsis cyclura, Per. and d'Orb. 

 (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 fins (lamelliform 

 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 uidamental 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. Spirulidee, 



Genus: Spirilla, Lam. (fig. 100, D). 

 Family 2. Bclcmnitidx. 



Genera : [Spirulirostra], d'Orb. (fig. 100, C) ; [Beloptcra], Desh. ; 

 [Belemnosis], Edw. ; [Coiwtcuthis], d'Orb. (fig. 100, A) ; [Acan- 

 thoteuthis], R. Wag.; [Bclcmnitcs], Lister, 1678; [Belemnitella], 

 d'Orb.; [Xiphoteuthis], Huxley. 

 Family 3. Sepiadse. 



Genera: Sepia, L. (figs. 92, 98, &c.); [Bclosepia], Voltz ; Coeco- 

 teuthis, Owen. 



