668 



MOLLUSCA. 



and the ItECAPODA, or ten-arraed Cuttle-flsli. The Oc(o_pod tribe consists of the two families Testacea sind. Nuda ; 



to the former belongs the genus Argmmuta, with (perhaps) the fossil genus Bdlerophon ; to the latter the genus 



Octopus, represented in the preceding figure (2) as creeping on 

 the shore with its mouth downwards. The Decapod tribe, which 

 includes the greater proportion of the existing Cephalopoda, is di- 

 TidHd into four families ; — the Teuthidoj or Calamaries, including the 

 genera Loligo, SepioteutkU, Onijchoteuthis, Scpiola, Cratichia, and Loli- 

 gopsis (Fig. 3) ; the Sepiad-x, or common Cuttle-fish, comprised under 

 the single genus Sepia ; the Sph-ulida:, consisting of the single genus 

 Spirala, which is a Decapodous Ccphalopod, ■\i'ith an internal spiral 

 chanibered shell, furnished with a siphun ; and the BeUrmnitidcv, a 

 group known only by fossil remains, but determined from these to 

 have been Decapodous Ccphalopods, possessing conical chambered 

 shells of which a description has been given in the text. From cer- 

 tain markings on the surface of these shells, and from the fact that 

 distinct remains of an ink-bag have been frequently met with in the 

 last or largest chamber of the cone, it has been argued that, not- 

 withstanding the strong resemblance of the shell to that of many 

 genera allied to the true Nautilus and belonging to the Tetrahran- 

 chiaU- group, the animal must have been Dibranchiate, and must have 

 included the shell, together with its massive sheath, in the same 

 manner as the Cuttle-fish includes the "pounce-bone." The Nau- 

 tilus possesses wy ink-bag, its power of completely withdrawing the 

 body into its shell rendering such a means of protection unneces- 

 ^^" sary ; and the ink-bag seems to be wanting in the several fossil 



Fig 3 — i ii ~,p^is. genera, whose shells bear a strong resemblance to that of this genus. 



If, then, the ink 1 ig le peculiar to the Dibranchiate order, and its presence indicates the general organi- 

 zation of that order, the Belemnite must have belonged to an animal more or less closely allied to the Sepvi. 



The justice of this view has been made evident by the recent discovery of specie 



mens of Belemnite, in which the soft parts of the animal are so well preserved 



as to enable their form and general structure to be distinctly traced. From 



these it has been ascertained that the arms were furnished with hooks, as in tlie 



Oni/chotattMs ; and that the body had a pair of small lateral fins, situated at 



about the middle of its length. From the weight of its dense internal shell, the 



Belemnite may be supposed to have commonly maintained a vertical position ; 



and, as its chambered portion was provided with a sipbuncle analogous to that 



of Naat'das, the animal probably had the power of ascending and descending in 



the water with f;icility. It would rise smftly and stealthily to fix its claws iu 



tlie belly <jf a fish swimming at the surface above ; and then, perhaps, as swiftly 



dart down and drag its prey to the bottom and devour it. "We cannot doubt 



that, like the hooked Calamaries of the present seas, the ancient Belemnites were 



the most formidable and predaceous of their class. [See Profc'ssor Owen's Me- 

 moir on the Belfiranite iu the Philosophical Transactions for 1844.] 



Order II. — TETEAEHANcni.iTA. The Ccphalopodes of this order are provided 



with a large L-xtL-rnal univulve shell, symmetrical in form, straight, or convoluted 



on a vertical phme. and divided by a si;ries of partitions into numerous cham- 

 bers, of which the last-formed is the largest, and alone contains the body of the 



animal ; a dilatable and contractile tube or siphon is continued from the pos- 

 terior part of the animal through all the partitions and chambers of the shell ; 



but the attachment of the shell to the body is eft'ected by means of two strong 



lateral muscles, wliich are inserted into the walls of the last chamber. The arms 



are very numerous, short, and hollow, each containing a lung, slender, retractile 



tentacle ; they are destitute of suckers. The head is provided with a large flat- 



ti-iiL-d ili^ik, which, besides acting as a defence to the orifice of the shell, serves 



iilsii, ill all probability, as an organ for creeping along the ground, like the foot 



in the Gasteropods. The jaivs of the Tetrabranchiuta are strengthened by a 



dense, exterior, calcareous coating, and have thick dentated margins. The ci/cs 



areijeduncutated, and of a simple structure like those of the Gastero]MHl;i. There 



is no organ of lirayi.,),-j. The giUs are four in numl.ier, and \vitliitiii lo'aiirhial 

 Tlie eiiTulatiiig system is provided with but one \'eiitricle, ■\^ili(■ll ig 

 , ><r propels arterial blood. There is no ink-bag. The inferior parietes 

 iiniel are ilivided longitudinally. 



TETFiAnB^ANCiiiATE order, the only existing representative is tbe genus 

 , ;Fig. Ti,) whose general organization has been described in the previuus aceount of the gruup. The 



