LOCALIZED STAGES IN DEVELOPMENT. 137 



cari'ies in itself an epitome of the ontogeny of the septa in the individual and concur- 

 rently in the phylum. 



Cyclolobus oldhamt. In the suture line of Cydolohus oldhami Waagen, one of 

 the simpler Triassic ammonites (Pig. 14), the saddle nearest the mnbilicus is almost 

 entire. Passing outward, toward the ventral border, the saddles become progressively 

 more complex, first slightly dentate, then more so, and finally twice dentate in the outer- 

 most saddle. Similarly the lobes next the unrbilicus are entire ; passing outward, 

 slight marginal saddles appear in the eighth lobe. In the fifth and sixth lobes these 

 saddles become roundly indented, and in the first lobe are comparatively complex. 



CVC'LOLOUUS OLDHAMI. 



14. Showing suture simple toward umbilicus anil progressively more complicated passing to ventral border, Triassic. 

 Zittel ('70-90). 



Cladiscites TORNATU.S. In Chi.di.^cites tornutus (Bronn), from the European Trias 

 (PI. 25, fig. 122), the fifth saddle nearest the umbilicus is dentate. The fourth saddle has 

 four short, lateral, branched lol)es; passing outward the septum becomes more complex 

 until the first saddle is dendritically branched. The fifth lobe has two slight marginal 

 denticulations ; in successive lobes toward the ventral border the marginal saddles increase 

 in number, and become dentately notched. 



PnrLLOCERAS NiLSSONi. Phylloccras n'dsmnl (Heb.), from the upper Lias of 

 Europe, has a very complex septum (PI. 25, fig. 123). The ninth saddle nearest the 

 umbilicus is (juite simple, being slightly dentate. In successive saddles, passing outward, 

 the dentations increase in depth and become lobate, until in the third saddle they are 

 slightly twice dentate. The second saddle is still more complex, but the first saddle is 

 somewhat simpler. The ninth lobe nearest the umbilicus is the simplest. In the sixth 

 lobe a small "entire marginal saddle appears. In the second lobe there are numeious 

 marginal saddles, part of which are dentate. 



TiiiARTHRUS BECKU. Bcecher ('96) from his studies of the ajjpendages of Tr'wr- 

 thrus Jicckii Green, of the lower Silurian, has shown that the anterior lind)s are long and 

 filiform. Passing posteriorly the limbs are progressively broadened, until those at the 

 posterior portion of the body are relatively very broad and resemble the limbs of 

 phyllopods and also the limbs of many crustacean embryos. As the anterior segments 



