24 EXTERNAL SHELL. 



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

 multilocular shells is encompassed with great difficulties, owing 

 to the variability of their characters ; in fact the synonymy 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. 

 Bickniore exhibited fifteen shells of Naulilttx PominUu*. of 

 various sizes, from one which measured live-sixths of an inch 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 geo- 

 logical ages ; and the Nautilus Pom/nliim at the different stages 

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



The body chamber is always very capacious ; more than double 

 the size of the combined air-chambers in Xaufi.fn* Po)npiti.n*. it 

 includes in some Ammonites more than an entire whorl of the 

 shell. The margin of the aperture, somewhat sigmoid and 

 simple in Nautilus, has projections or extensions in some fossil 

 species; and in Phragmoceras and (Jomphoceras 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. l>arrande thinks that the 

 neck was enclosed in the upper part of the aperture, the hit era 1 

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

 But there is reason to believe that the fossil Annnonit.es pos- 

 sessed a more effective method of closing their aperture ; namely 

 a horny or shelly <>/><'rrnltun. In the Nautilus the union and 

 expansion of the two dorsal arms forms a disk or so-called hnnd^ 

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

 Ammonites (probably secreted by these dorsal arms) there 

 appears to have been a (rue operciilum ; at least, opercillar-shaped 

 bodies of which many species have been described a re constantly 

 associated with, and frequently within the body chamber of the 

 Ammonites. The true nature of these shelly or flexible horny 



