THE SHELL. 37 



the form but merelj^ the external surface of the shells. Some 

 fossil Nautili, for instance, striated when j'-oung, become smooth 

 at a later period, whilst others smooth when young are striated 

 or ribbed when adult. So in the Ammonites, the juvenile shell 

 is always smooth, but in the course of growth, tubercles, ribs and 

 strife appear, and develop until the animal has become adult ; 

 after this period degeneration takes place, the ornaments gradu- 

 ally disappear in the successive growths, and in old age the 

 surface of the outer whorl ma}- be as smooth as in youth. In 

 Ammonites of the same species two forms of shells may be 

 observed, one much compressed, the other swollen ; and it is 

 reasonable to conjecture from analogy with living species that 

 the sexes are thus indicated to us, the swollen shell being 

 required for the ovary of the female. 



It will be seen from the above that the study of the species of 

 multilocular shells is encompassed with great difficulties, owing 

 to the variability of their characters ; in fact the synonj^my of 

 the species of Ammonites has been greatly increased in conse- 

 quence of several names being given to the same species at 

 different periods of its growth. 



The living Nautilus also, undergoes a change of form. At a 

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

 Bickmore exhibited fifteen shells of Nautilus Fompilius of 

 various sizes, from one which measured five-sixths of an iiich by 

 one inch and one-sixth in its two diameters, to one measuring 

 two and five-sixths inches by three and three-fourths inches in its 

 two diameters. The smaller ones are so loosely coiled that it 

 is possible to look between the coils. These young specimens 

 therefore represent the loosely coiled Nautiloids of former 

 geological ages ; and the Nautilus Pompilius at the different 

 stages of its growth is an epitome of the whole group. 



The body-chamber is always verj' capacious ; more than double 

 the size of the combined air-chambers in Nautilus Pompilius 

 (iv, 63)', it includes in some Ammonites (ii, 21, 28), more than 

 an entire whorl of the shell. The margin of the aperture, some- 

 what sigmoid and simple in Nautilus, has projections or 

 extensions in some fossil species ; and in Phragmoceras and 

 Gromphoceras (xxx, 100-1), the aperture is even so considerably 

 contracted as to have led to the supposition that the animal was 

 not able to withdraw its head and tentacles within the shell. 



In these curious silurian forms M. Barrancle thinks that the 

 neck was inclosed in the upper part of the aperture, the lateral 

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

 But there is some reason to believe that many of the fossil 

 Ammonites possessed a more effective method of closing their 

 aperture ; namely, a horny or shelly operculum. In the Nautilus 

 the union and expansion of the two dorsal arms forms a disk or 



