l'l K.\ I'KK.NAI, Ml Kl.l.. 



It will IK- seen from the above that tin- study of tin- specie 

 multilocular sht-lls is encompassed \villi great dillicultie^. owinij 

 to tlir variability of their characters; in fact t he synonymy of 

 the species of Ammonites IIMS been greatly increased in conse- 

 <iucnee of several names being given to the same specie^ ;it 

 different periods of its growth. 



Tin? living Nautilus also, undergoes a change of form. At a 

 recent meeting of the Boston Society of Natural History. 1'rof. 

 Biekmore exhibited (ifteen shells of \anfiln* /'(nn/tHiu.*. <>f 

 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 ^co- 

 logical aircs; and the Aw"//7>/x rom/iiHux at the different sts 

 of its growth is an epitome of the whole group. 



The body ch(imln'r is always very capacious ; more than double 

 the size of the combined air-chambers in X(infi/n.< /'<nn/>ilin*. it 

 includes in some Ammonites more than an entire whorl of the 

 shell. The margin of the npertnre. somewhat sigmoid and 

 simple in Nautilus, has projections or extensions in some fo--il 

 species; and in Phragmoceras and Gomphoceras 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. P>arrande think^ that the 

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

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

 lint there is reason to believe that the fossil A mmonites p. .-- 

 d a more effective method of closing their aperture : namely 

 a hornv or shellv <>/', -m/i/ in. In the Nautilus the union and 

 expansion of the two dorsal arms forms a disk or so-called /i<mtl, 

 b\- 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 1 rue opcrciilum : at least operciilar-shapcd 

 bodies of which many species have been described are constantly 

 iated with, and IVe|Uently within the body chamber of the 

 Ammonile->. The 1 rue nature of these shelly or flexible horny 



