668 



MOLLUSCA. 



and the Decapod a, or ten-anneJ Cuttle-fish. The Octopocl tribe consists of the two families Testacca v^nd Xuda ; 

 to the former belongs the genus Argonauta, with (perhaps) the fossil genus Belleropkon ; to the latter the genus 



Octoinis, 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- 

 vided into four families ; — the Teuthidai or Calamaries, including the 

 genera Loligo, Sepioteuthis, Onychoteuthis, Sepiola, Cranchia, and Loli- 

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

 the single genus Sepia ; the Spirulidce, consisting of the single genus 

 Spinda, which is a Decapodous Cephalopod, with an internal spiral 

 chambered shell, furnished with a siphon ; and the BelemnitidcE, a 

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

 have been Decapodous Cephalopods, 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 Tetrabran- 

 chiate group, the animal must have been Dibrancliiate, 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 iio 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 LoLiGop.'iis. genera, whose shells bear a strong resemblance to that of this genus. 



If, then, the ink-bag be peculiar to the Dibrancliiate 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 Sepia. 

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

 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 tliat the arms were furnished with hooks, as in the 

 Onychoteuthis ; 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 siphuncle analogous to that 

 of Nautilus, the animal probably had the power of ascending and descending in 

 the water with facility. It would rise swiftly and stealthily to fix its claws in 

 the belly of 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 Professor Owen's Me- 

 moir on the Belemnite in the Philosophical Transactions for 1844.] 



Order II.— Tetrabranchiata. The Cephalopodes of this order are provided 

 with a large external univalve shell, symmetrical in form, straight, or convoluted 

 on a vertical plane, and divided by a series 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 effected by means of two strong 

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

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

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

 tened disk, which, besides acting as a defence to the orifice of the shell, serves 

 also, in all probability, as an organ for creeping along the ground, like the foot 

 in the Gasteropods. The jaws of the Tetrabranchiata are strengthened by a 

 dense, exterior, calcareous coating, and have thick dentated margins. The eyes 

 are pedunculated, and of a simple structure like those of the Gasteropoda. There 

 is no organ of hearing. The giUs are four in number, and without branchial 

 hearts. The circulating system is provided with but one ventricle, which is 

 systemic, or propels arterial blood. There is no ink-bag. The inferior parietcs 

 of the funnel are divided longitudinally. 



Fir.. 4. 



■ Belemnite RE":TOREn; 

 a, a, outer chamber of cone . *, cham- 

 bered portion ; c. spathose guard ; d, ink- 



Of the Tetrabranciiiate order, the only existing representative is the genus hooked'krmsT'XfunneL ■^' ^"' ' ^' ^' 

 Nautilus, ;Fig. 5,) whose general organization has been described in the previous account of the group. The 



