V *V:^ >S c*/ v < / ceo s 



MOLLUSCA. CEPHALOPODA 283 



resembles Orthoceras, but the calcareous protoconch and slender 

 marginal siphuncle seem to connect it more closely with the Am- 

 monoids. The wall of the phragmocone (sometimes termed the 

 conotheca) is very thin, and in well-preserved specimens the upper 

 part is found to be produced in front into a large laminar expansion 

 (fig. 121, d) ; this pro- 

 longation is known as c " e J 

 the pro-ostracum, and </ y '-j (V <^ 

 corresponds to the 'pen' *fs \s v«* y\^ 

 of the squids. The head «-^ -^ >/ >/ 

 of the Belemnite was im- 

 mediately in front of the 



pro-ostracum. The suck- a v - ' ,N ^^^"'^S* ' t r 

 ers on the arms were *4£ r ~ i ^> v . I /" 



provided with horny /,/ ''- ^ v*s 



hooks, which are some- ' (^ L 



times preserved fossil Fig 122 A Belemnites. Lias, Lyme Regis. 

 (fig. 122 J) ; there was a Showing hooks indicating the presence 



double row of hooks on f eight arms (a— h). xf. 



each arm, but only eight 



double rows have yet been found in any specimen ; two other 

 arms, with or without hooks, may have been present. The ink-sac 

 and mandibles have also been found in some specimens. The 

 probable positions of the guard, phragmocone and pro-ostracum in 

 the body of the Belemnite are shown in fig. 122, from which it is 

 seen that the guard formed a relatively small part of the entire 

 length of the animal. The 'genus' Belemnites is founded mainly 

 on the characters of the guard, and includes an enormous number 

 of species. Probably if the soft parts were known, they would 

 show such differences as to indicate a number of distinct genera. 

 Lower Lias to Upper Cretaceous. Ex. B. acutus, Lias ; B. oweni, 

 Oxford Clay, etc. ; B. hastatus, Oxford Clay. 



Belosepia, found in the Eocene, is related to Belemnites, but the 

 guard is considerably reduced in size, and the septa curve forward 

 from the broad siphuncle towards the pro-ostracum. Spirulirostra, 

 from the Eocene and Miocene, is another allied form ; it possesses 

 a small guard ending in a point, and the first part of the phragmo- 

 cone is coiled. In Sepia (Eocene to Recent), the laminae which 

 form the main part of the shell, are believed to represent the septa 



