24 EXTKRNAh f^IIKLI.. 



It will Ite- seen from the above^ tliut tlic study of tliu species of 

 nuiltiloeular shells is encoin[)asse(l with great (lillleiiltics, owing 

 to the vaiialiility of tlieir characteis ; in fact the synonymy of 

 the species of Ammonites has been greatly increased inconse- 

 quence of several names being given to the same species at 

 different periods of its growth. 



'I'he living Xautilus also, undergoes a change of form. At a 

 recent meeting of the Boston Society of Natural History. Prof. 

 Bickmore exhibited fifteen shells of Natifilas Pompiiius. of 

 various sizes, from one Avhich measured five-sixtlis of an ineh by 

 one inch an<l one-sixth in its two diameters, to one measuring 

 two and (ive-sixths inches by three and three-fourths inches in 

 its two diameters. The suialler ones are so loosidy coiU'd th;il it 

 is possible to loolv between the coils. Tlu'se young si)ecinu'ns 

 therefore represent the loosely-coiled Xautiloids of former geo- 

 logical ages; and the Naiifihis Pom]iiliii^ at the diiferent stages 

 of its growth is an epitome of the whole group. 



The hofhj chauiher is always very capacious ; more than doul)le 

 tile size of tlie eoml)iued air-cliaml)ers in Xtm/ihis Poiiijii/iiis^ it 

 includes in some Ammonites more tiinu an entire whorl of the 

 shell. The margin of the aperture, somewhat sigmoid and 

 simple in Nautilus, has projections or extensions in some fossil 

 si)ecies; and in Phragmoccras and Gomphoceras the aperture is 

 even so considerably contracted as U) have li-d to the supposition 

 that the animal was not able to witlidrnw its head and tentacles 

 within the shell. 



In these curious silurian forms M. Harraude thinks that the 

 neck was enclosed in the U[)i)er part of the aperture, the lateral 

 lobes giving passage to arms, and the lower lobe to the funnel. 

 But there is reason to believe that the fossil Ammonites pos- 

 sessed a moi'c effective method of closing their a[)erture ; namely 

 a horny or shell}' operculum. In the Nautilus the union and 

 ex|)ansion of the two dorsal arms forms a disk or so-called hood^ 

 by which the animal may close the aperture of tin; shell, and in 

 Ammonites (probably secreted b}' these dorsal arms) there 

 ap|>ears to h;ive l)een a true operculum ; at least opercular-sliaped 

 boflies of which many spi'(ties have been described are constantly 

 associated with. :iu<I fi-ctpienlly wiMiin thi^ l)ody chamber of the 

 Ammonites. The true nature of these shelly or flexible horny 



