16 BRITISH FOSSILS. 



The hooks of one arm have remained in position, and are arranged in 

 two rows, and opposite to one another. One hook is so imbedded in 

 the matrix as to expose its outer or convex side. In this, as in the 

 lateral position, the base is much .wider than the shaft of the hook. 



The guard is not preserved in any of the preceding fragmentary 

 specimens, while the ink-bag is but indistinctly traceable in the entire 

 one first described. But any hypercritical doubt that might remain 

 as to the possession of an ink-bag by a true Belemnite, must be removed 

 by Mr. Day's specimen of Belemnites elongatus represented of one-half 

 the natural size in Plate I., fig. 2, which exhibits the guard and phrag- 

 mocone complete, with a large and full ink-bag in situ. The ink-bag 

 is pear-shaped, and tapers off to its duct. The length from the ex- 

 tremity of this to the base of the bag is 1 4 inch, the widest part of the 

 bag measuring 0*55 of an inch. The shell from the apex to the mouth 

 of the phragmocone is 5 35 inches long. The guard from its apex to 

 the point at which it begins to expand over the phragmocone measures 

 about 2\ inches, and is 25 of an inch wide at widest. 



These measurements may enable one to form a rough estimate of the 

 size of guard which appertained to any detached ink-bag, and rice versa. 



I have not been able to make out more than six or seven arms in 

 any specimen, nor has any exhibited traces of elongated tentacula, 

 though the shortness of the arms which have been preserved would 

 lead one to suspect their existence. The hooks in the middle of the 

 length of each arm seem to have been largest ; those at the ends of the 

 series, especially at the base, smallest. 



The foregoing descriptions demonstrate that certain true Belemnites 

 were provided with hooks upon their arms ; horny beaks ; large ink- 

 bags ; and with a pro-ostracum (into which iridescent, nacreous, shelly 

 matter entered more or less largely) prolonged as a broad spatula te plate 

 along the whole length of the dorsal region of the mantle, and pro- 

 duced laterally and interiorly, for an unknown distance, along the lateral 

 and ventral regions of the body. 



But it by no means follows that all Bclemnitida were provided with 

 a pro-ostracum of similar form and character. On the contrary, it 

 appears to me to be certain that there were at least two other kinds of 

 pro-ostracum in this family. 



Thus the Oxford Clay Belemnite, described by Mantell (Phil. 

 Trans., 1848), under the name of attemiatus, a name which appears, 

 like B. Owenii, to be only a synonym of B. Puzosianus (D'Orbigny) 

 has a pro-ostracum which was very thin and apparently horny, or 

 imperfectly calcified, in the dorsal region, and was supported laterally 

 by two thin calcareous bands, or pillars, which, inferiorly, expand upon 

 the conotheca. 



A third very distinct type of pro-ostracum is exhibited by that 

 i-emarkable Belemnitoid originally figured and described under the name 

 of Orthocera elongata, by Sir Henry De la Beche,* who says in a note 

 (1. c., p. 28), " I have ventured to class this specimen as an Orthocera, as* 

 " it possesses more of the character of that genus than of the Belemnite, 



* On the Lias of the coast in the vicinity of Lyme Regis, Dorset. Transactions 

 of the Geological Society, ser. 2nd, voL ii. (1829), PI. IV., fig. 4. 



