﻿BRITISH FOSSIL CEPHALOPODA. 17 



found that in many points the Tetrabranchiata represent an early stage of the 

 Dibranchiata, as, for example, in the structure of the eye, the position of the auditory 

 organ, and the disposition of the tentacles. This fact is of great interest in connec- 

 tion with their geological distribution, for that order which is most embryonic in 

 structure is also earliest in time ; for we cannot suppose that, with all the imper- 

 fection of the geological record, Belemnites could have existed during the Silurian 

 period and not have left a single indication in the rocks. 



The structure of the shell and of the organs immediately connected with it is so 

 essentially connected with the interpretation of fossil forms, which, in their turn, 

 throw light upon the recent one, that it is necessary to consider it separately. 



The Shells of the Nautiloids and their Terminology. 



As the description of the soft parts has been restricted to that of the single 

 representative of the Tetrabranchiates, so must the features of the shell be only 

 here noticed so far as they are found in the Nautiloidea, which alone occur in Silurian 

 deposits, and the characters of the Ammonitoidean shells will be reserved till their 

 British fossil representatives come to be described. 



In relation to the shell, we have to consider : — 1. The internal structure. 2. The 

 external form. 3. The commencement. 4. The body-chamber. 5. The aperture. 

 6. The ornaments. 7. The septa. 8. The siphuncle. 9. The internal marks. 



1. The Internal Structure. 



The structure of the shell of the recent Nautilus pompilius has been described by 

 Dr. Carpenter in the ' Report of the British Association for 1847,' and figures have 

 more recently been given by Hyatt. 1 In the external shell two distinct layers have 

 generally been recognised (PI. II., fig. 1, a, b), and Hyatt describes also a third or 

 lining layer, which will be noticed presently. Of the first two, the outer one is 

 usually called the porcellanous layer, because it is opaque. As seen in a thin 

 section parallel to the surface, it presents a speckled appearance, which, on raising the 

 magnifying power, is seen to be due to a number of transparent crystalline particles, 

 set in a dark and opaque ground (see PI. II., fig. 6) ; these particles have irregular 

 outlines, and the crystalline structure shows a radiating arrangement. They are so 

 closely aggregated as to leave the darker substance as lines only with occasional 

 spaces. The size of the particles is largest towards the inside where the two layers 

 unite, and their radiating structure is there more distinctly seen. The colours of 

 the shell are produced in this layer, and they do not arise from the increase of the 

 intermediate substance, but are due to the coloration of the crystalline particles 



1 " Fossil Cephalopoda of the Museum of Comparative Zoology — Embryology." — Bull, of Mus. 

 Comp. Zool., vol. iii. No. 5, 1875. 



D 



