CEPHALOPODA. 95 



believed to be filled with air and to act as a float. The exact 

 fniiction of the sipliuiicle is still unknown. The members of 

 this order possess no ink-bag. There is only one living tetra- 

 brauch — the Nautilus, and this is one of the few genera which 

 liave existed at every period of the world's history. Toward 

 6,000 species have been named, but nniny of them may be tlie 

 internal shells of dibranchs. This order came first, and culmin- 

 ated as the more highly organized Cuttles made their appearance. 

 Tiie four-gilled type began in the straight OrthoGeras and half- 

 coiled Lituites of the Silurian, culminated in the complex Am- 

 monites of the Jurassic, and declined in the Ci'etaceous through 

 the half-coiled forms and the straight Baculites. The straight 

 tetrabranchs must have lived habitually in a neai'ly vertical posi- 

 tion, while the discoidal genera would also creep over the sea-bed 

 with their chambers above them. 



FAMILY AMMONITID^. 



This is the most remarkable family of Secondary niollnsks. 

 Some twenty or more genera, and several thousand species, have 

 been identified. They range from the Devonian to the Creta- 

 ceous, inclusive, becoming totally extinct at the close of the Rep- 

 tilian Age. The family includes the following principal genera 

 in the order of their appearance : Goniatites^ ATUTnonites proper, 

 Ancyloceras and Ilelicoceras, Crioceras, Toxoceras, Scaphites, 

 Ilamites, Baculites and Ptychoceras^ and Turrilites. The type 

 began in the discoidal Goniatites, culminated in the compactly 

 coiled and elaborate Ammonites, and expired in the half-uncoiled 

 forms as Scajphites, the spiral Turrilites, and the straight Bacu- 

 lites. 



This division of chambered shells is distinguislied from the 

 succeeding families b}' more varied and more highly ornamented 

 forms, with crumpled septa, lobed sutures, and dorsal siphuncle. 

 The shells are most beautiful when of middle growth, the orna- 

 mental charactei's being less developed in the young, and lost in 

 the very old ; and on account of this change of form and orna- 

 mentation with age, the old classification of von Buch and 

 d'Orbigny is no longer satisfactory. Efforts have been made by 

 several naturalists to systematize the family. Prof. A, Hyatt 



