1 I KXTKKXAL Mil KM.. 



It will In- seen from the above that the study of tin- species of" 

 multilocular shells is encompassed with great difficulties, owing 



to the variability of their characters: in l';iet the synonymy of 



the species of Ammonites has been greatly increased in conse- 



(jiienee of several names being given to the same species at 

 different |)erio<ls of its growth. 



The living Nautilus also, undergoes a change of I'orin. At ;i 

 recent meeting of tlu 1 Boston Society of Natural History. I'rof. 

 Bickmore exhibited fifteen shells of \<nitil//x /'o/y?/>/7///.<. of 

 various sizes. from one which measured live-sixths of an ineh by 

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

 two and live-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 au'es; and the Ndiiiilux Pompiliits &t the different stages 



ol' its oTowfh is an epitome of the whole group. 



The IKH!I/ C/KI HI/XT is always very ca paeious ; more than doulile 

 the size of tin- combined air-chambers in \<t til II u* /'o////*////^. 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 I'hragmoceras and (Jomphoceras t he apert tire is 

 even so considerably cont racted as to have led to the supposition 

 that the animal was not aide to withdraw its head and tentacles 

 within the shell. 



In these curious silurian forms M. l>arraude thinks that the 

 neck was enclosed in the upper part; of the aperture, the lateral 

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

 lint, there is reason to believe that the fossil A mmonites pon- 



d a more effective nn't hod of closing their apert lire ; namely 

 a horny or shelly '>/'/>>!/ i/m. In the Nautilus the union and 

 expansion of the two dorsal arms forms a disk or so-called ///>//, 

 bv 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 t rue opcreulum ; at least opercular-shaped 

 bodies of which many species have been described are constantly 



M-iated with, and frequently within the body chamber of the 

 Ammonites. The true nature of these shelly or tlexible horny 



