88 BLUE CHALK MARL. 



LS. Nautilus inequalis. Tab. xxi. fig. 14, 15. {Min. Conch, tab. xL 

 2, S.) Involute, spheroidal, urabilicate, aperture obovate ; septa entire, 

 slightly concave, the inner septa deeper than the outer ; siphuncle placed 

 near the inner margin. 



The specific name of this elegant shell is taken from the remarkable 

 structure of its septa, wliich diminish in depth as they approach the aper- 

 ture ; while, in every other species, they increase in size with the age 

 of the animal. 



Tab. xxi. fig. 14. Front view of a cast from Norhngton. 



— fig. 15. Lateral view of the same specimen. This cast is com- 

 posed of indurated marl, impregnated with iron ; remains of the shell 

 changed into carbonate of hme, form the curved hues which mark the 

 division of the chambers. 



14. Belemnites Listeri. Tab. xix. fig. 17, 18, 23. 



minimus ; Lister. Hist. Anim. AnglicE, p. 228, fig. 32. 



Subfusiform, cylindrical, with one shght longitudinal sulcus; apex 

 pointed ; siphunculus central, extending through the alveolus to the apex 

 of the spathose part. 



The form of this beautiful little belemnite varies considerably ; 

 some of the specimens are fusiform, others gently taper towards the 

 apex ; some are perfectly cylindrical, and others contract suddenly. 

 Their constituent substance is a spathose crystallized carbonate of lime, 

 of a radiated structure, varying from a dark brown to a light amber 

 colour ; many of the specimens are nearly opaque, but the greater part 

 are pellucid. The largest example in my collection is 02 inch in 

 diameter, and 1 -3 inch in length. Upon the application of a slight force 

 in the direction of the sulcus, they separate longitudinally, and expose 

 sections of the chambered structure of the shell, with the siphunculus ex- 

 tending through the spathose substance to tlie apex. These fossils occur 

 in profusion in every locahty of the blue marl in Svissex ; and also in 

 Surrey, Kent, and Cambridgeshire. The same species is found at Stut- 

 gard *. 



* Geological Transactions, vol. v. p. 58. 



