21G 



INVEitXEHRATK ANIMALS. 



known, excejit that it niiiMt cfrtuinly .sevvi.- to UKiintaiii tlie vitality 

 of the shell. 



Of the fossil Tetridiraui-lniitii the most important are the Ortho- 

 ivi-atd and l\i& Ammonitrx. The Orlhorerotn played a very important 

 part iu the seas of the Paheozoie or Ancient-life ]ierii>d of the earth's 

 history, in which they apparently lilled the ]>taee now taken Ijy the 

 predaceons Cuttle-tishes. They anieed with the NnntHiis m having 

 a many-chambered shell, diviiled by cnrved partitions, perforated 

 by a tube or siphuncle. The shell, however, differed from that of 

 the XautUus in not being curved or coiled up, but in l_>eing straight. 

 In other nearly allied forms the shell was bent or even partially coiled 

 up, but never so completely iis in the true Xniitili. Many of the 



Fig. ].0r..— Tlic P.-irly Naiililus (Xniilihis ponipilivx). a Jlaiitlc ; (. Its ilnrsal fcild ; 

 (■ Hn.irt : ,. E)v; ( T.Mitiifl.'S ; ,/• Fuiiiid. 



Orthocei-ata were of sm.ill size, but some of them were colossal, shells 

 having been found of six o)- seven feet in length, and as thick as the 

 body of a m.-ni. 



The AitiiiKiiiitrx (lig. 15fi), with a. number of allied forms of varied 

 shapes and l>e.-iutiful structure, a])jiear t(. have taken the jilace of the 

 Naatilidii\ to a great extent, in the se:is of the Secondary jieriod ; 

 at which time, too, ])iliraneliiate ( 'eiilialojjods first made their ap- 

 pearance. The tiiie Anjiiionites I'esembled the Xaiitihix in having 

 a many-ch;inil]eieil shell, which was coiled u|i into a. sjiiral, but the 

 jiositioii of tin' siphuncle was different, and tlu! ])artitions or .se))ta 

 Ijctwccn tlic \:irious chambers (jf the shell were wonderfully folded 



