K\o\vi,i:nr,i:. 



SEr'TEMni-.R. 1912. 



quite fully established that the 



effect of external conditions, once 



rather underestimated, has a dis- 

 tinct influence upon the course and 



nature of evolution. That the 



.\ininonites had similar modes of 



attachment of the animal to the 



shell is abundantly illustrated 



by the testimony of numerous 



S[)ecimens exhibiting the same 



characteristic. 



The composition and structure 



of the shell in Nautilus and its 



early ancestors has also remained 



remarkably uniform. Thus, Figure 399. 



in the fossil forms we find the Straight-shelled 



shell consisting of two Ia\ers, Cephalopod, 



an internal nacreous iridescent Orthoceras 



layer formed internally, and an ^'''^'^ "reduced). 



external thicker layer formed by the mantle edges, 



whilst in addition to these, which are superposed in 

 an imbricate manner like 

 tiles, there was a " black 

 layer," which was formed 

 externally by the hood, 

 representing the animal 

 matter which was secreted 

 by tiiat organ. The same 

 "black layer" is found 

 within a double wall on 

 either side of the inner and 

 ■ outer layers in the earlier 

 parts of the shell before 

 the last septum, as seen in 

 the diagram. It occurs out- 

 side only in the volute part 

 of the shell because there 

 the wall is not duplicated 

 as it is where the walls 



of newer and older chambers were in contact. The 



presence of this "black layer" between the two 



other layers placed on either side 



is demonstrated by the decalcification 



of a portion in acid, when there 



remains a certain quantity of organic 



matter representing the former " black 



layer." And the formation of shells is 



now held to be due to excretion and 



formed by the mantle or arms of the 



animal, by the building up of carbonate 



of lime upon a membrane of organic 



matter of a chitinous nature. This is 



the condition of the " black layer." 



The periostracum of many bivalves 



and univalves is formed by the 



mantle, and is the first-formed part 



of the shell, the calcareous part and 



coloured layer being deposited next 



Figure 400. 

 Curved-shelled Cepha- 

 lopod, Cyrtoceras type 

 (reduced). 



Figure 401. 



Loosely-coiled Cepha- 

 lopod, Gyroceras type 

 (reduced). 



Figure 403 



appearing. The operculum of 

 some species is deposited by 

 the foot, and is not homologous 

 with the last or the "black 



layer." 



It is a point of some interest 

 that the " black layer " which 

 forms the inner dividing layer of 

 two contiguous, but opposite, 

 shell-walls, should be preserved in 

 fossil forms of Xautilus in as 

 perfect a manner as that which 

 is seen so clearly in living ex- 

 amples of the Pearly Nautilus. 



This is well-illustrated in the 

 accompanying photograph of 

 the Liassic Xautilus striatus 

 Sow. (see Figure 398 A), and 

 a young example of the 

 living Pearly Xautilus. In young forms of the 

 latter the deposit is a shin\- black opaque laver 

 which is remarkabh' smooth 

 and jet-like. But in older 

 examples there are numerous 

 plicae or folds coinciding 

 with the gradual encroach- 

 ment of the hood upon 

 the coloured shell surface, 

 which is the ventral sur- 

 face continued, and lies next 

 to the dorsal lobe of the 

 hood which would be against 

 the dorsal side of the shell 

 if it were continued bevond 

 the last septum, which is 

 not the case. These plicae 

 are reproduced in the fossil 

 form. Upon the surface 

 also may be observed both 



in the recent Nautilus and in this Liassic species, 

 numerous excrescences, which represent the papillae 

 on the hood. Such a remarkable re- 

 semblance between species so remote 

 in relationship is truly extraordinan,-. 

 But in these two specimens again 

 we have a further interesting feature, 

 fur in the Liassic species below the 

 central si[5huncular impression is a 

 similar, but depressed, ring-like mark- 

 ing, which represents the annular or 

 median dorsal lobe. It is not often 

 to be detected in the last septum of 

 Wnifiliis pompilius (see Figure 398 B), 

 but is represented in earlier septa, and 

 in some young forms by the little pit- 

 like hollow here seen in Xaiifilus 

 striatus. 



The occurrence of two 



Figure 402. 

 Closely-coiled Cepha- 

 lopod, Nautilus type 

 (reduced). 



but, unlike the "black layer," the Section through an early stage features in the Liassic and present 

 periostracum is only a protection, and ^«"'''"* '° ^^'"'^ "''"•"'' 



central 

 black laver (enlarged). 



interestine 



da\- Nautilus 

 make-shitt as it were, and not a ' indicates the presence in both of 



permanent part of the shell, not being absorbed into annular muscle. No figure of the " black laver ' 

 the interior in a coiled species but eventually dis- a fossil species has hitherto appeared. 



It 



