§ 28. AMMONITES OF THE CHALK. 335 



formations, and certain species are restricted to particular 

 rocks. Thus, for example, the chalk marl of Sussex abounds 

 in two species (Lign. 63, figs. 7, 9), which very rarely 

 occur either in the chalk above, or in the gait below ; and 

 in every locality of the marl in England, and on the Conti- 

 nent, these have been met with.* The membranous tube 

 of the siphuncle sometimes occurs in a fossil state, as 

 may be seen in this ammonite from the chalk marl near 

 Lewes, in which a large portion remains ; the black sub- 

 stance of these tubes has been analyzed, and found to 

 consist of animal membrane, permeated by carbonate of 

 lime.t 



Ammonites vary in size from a few lines to twelve or 

 fourteen feet in circumference ; at low water on those 

 parts of the Sussex coast where the chalk forms the base 

 of the shore, enormous specimens are often seen imbedded. 

 In some of the marls and clays of the chalk, hundreds of 

 ammonites occur in clusters. 



28. Turrilite, hamite, &c. — Baculites, Turrilite.% Ha- 

 wites, Scaphites, and other genera of multilocular shells, 

 abound in the chalk-marl, gait, and greensand. The turri- 

 lite (Lign. 63, Jig. 2), may be described as an ammonite 

 twisted in a spiral, instead of a discoidal form : and the 

 hamite (Lign. 63, Jig. 6), as a similar structure in the shape 

 of a hook, coiled up at the smaller extremity. These shells 

 sometimes attain a large size ; the turrilite before you, 



* See my Fossils of the South Downs, for figures of many chalk 

 Ammonites : also Mineral Conehology ; Geological Journal ; Medals 

 of Creation, vol. ii. p. 489 ; and M. D'Orbigny's Palseontologie. 



•f Dr. Prout is of opinion that the black colour has originated 

 from decomposition ; the oxygen and hydrogen of the animal 

 membrane having escaped, and carbon evolved, as happens when 

 vegetable matter is converted into coal, under the process of minerali- 

 zation. The lime has taken the place of the oxygen and hydrogen, 

 which existed in the pipe before decomposition. — Dr. Buckland's 

 Bridgewater Essay, p. 352. 



