BELEMNITELLA. 5 



The genus Belemnitella was separated from Belemnites by M. D'Orbigny, on account 

 of the slit down the front of the alveolus, and the vascular impressions on the back and 

 sides of the shell, which are wanting in Belemnites ; the latter character is most strongly 

 developed in M. mucronata and B. quadrata, and but faintly seen in B. lanceolaia and 

 B. plena. These characters seem a slight foundation for a separate genus; but the 

 division has been generally adopted. All the Belemnitellge, yet known, belong to the Chalk. 



The Beleranitellse may be divided into two sections, depending on the form and lining 

 of the alveolar cavity : in B. mucro?iata and B. lanceolaia the alveolus is a regular cone, 

 with a rounded base, of which the apex forms an angle of about 20 degrees ; the cavity is 

 lined by a smooth continuous shelly layer, which in the fossils is denser, and less 

 crystalline than the rest of the shell ; this cavity is filled by the phragmacone, and is 

 slightly marked by the edges of the septa. 



In B. quadrata the lower part of the cavity for about one third of an inch is in the 

 form of a cone, with a round base, of which the apex has an angle of about 20° ; this is 

 lined with a smooth shelly layer, and was evidently destined to receive the phragmacone. 

 The cavity then widens out suddenly, and becomes nearly square ; this upper angular part 

 has no special lining, but is rough, with irregular rings marking the growth of the shell ; 

 the anterior slit reaches a little way into the lower part of the cavity. It has been 

 suggested, that the upper angular part of the cavity may be due to decay ; but its form is 

 too constant and regular to be so explained. 



The Swedish species B. suhventricosa of Wahlenberg {B. mamillatus, Nills., B. scanice, 

 Blainv.) has a cavity of analogous form to B. quadrata, with an irregularly triangular 

 aperture above. M. Konig figured this species under the name of Paxillus belemnoideus 

 (Icones, No. 218), apparently regarding its peculiar form of cavity as a sufficient generic 

 character. But, as at present, we do not know the form of the phragmacone of either of 

 these species, we cannot estimate the importance of the difference in the form of the cavity. 



The vascular impressions are very differently developed in the different species of 

 Belemnitella ; they all have two depressions which extend down the sides of the posterior 

 part ; at the edge of the cavity these depressions are nearly as broad as the space between 

 them, they diverge slightly and become narrower in their course downwards, and termi- 

 nate near the apex of the shell, almost as near to the anterior as to the posterior side. 

 These two depressions are usually all that we see of the vascular markings on B. lanceolaia 

 and B. plena, and on worn specimens of the other species; but on well-preserved 

 specimens of B. mucronata and B. quadrata they are bounded on each side by an 

 impressed hue, from which spread off a number of branches, dividing and subdividing 

 with great complication, which cover the sides and front of the shell. 



Casts of the alveolar cavity of Belemnitellae are frequently found in the flint of the Upper 

 Chalk, which sometimes show more clearly the form of the cavity than can be seen in the 

 shells themselves ; the two woodcuts a and b are copied with Dr. Mantell's permission, 

 from the second edition of his ' Geological Excursions round the Isle of Wight,' p. 440 ; 



