35 8 MANUAL OF ZOOLOGY. 



duced into a horny plate or "pen," and lodged in a cylindrical 

 fibrous "guard." 111. Gen. Belemnites, Belemnitella, Belem- 

 nitheutkis, 



Fam. 5. Sepiada. 



Shell calcareous, consisting of a broad, laminar plate, termi- 

 nating posteriorly in an imperfectly chambered apex ("phragma- 

 cone "). 111. Gen. Sepia, Beloptera, Spirulirostra. 



Fam. 6. Spirulidce. 



Shell internal, nacreous, chambered, discoidal ; the whorls 

 separate ; a ventral siphuncle. Gen. Spirula. 



ORDER II. TETRABRANCHIATA. 



Animal with four gills ; arms more than ten, without 

 suckers ; no ink-bag ; shell external, chambered, and 

 siphuncled. 

 Fam. i. Nautilidce. 



Sutures of the shell simple; the siphuncle central, sub-central, 

 or near the concavity of the cui-ved shells, simple. 

 Sub-family Nautilida proper. 



Body-chamber capacious ; aperture simple ; siphuncle 

 central or internal. 111. Gen. Nautilus, Lituites, Trocho- 

 ceras. 

 Sub-famUy Orthoceratidtz. 



Shell straight, curved, or discoidal ; body-chamber small ; 

 aperture contracted; siphuncle complicated. 111. Gen. 

 Orthoceras, Phragmoceras, Cyrtoceras. 

 Fam. 2. Ammonitidce. 



Shell discoidal, curved, spiral, or straight ; body-chamber 

 elongated ; aperture guarded by processes, or closed by an oper- 

 culum ; sutures angulated, lobed, or foliaceous ; siphuncle ex- 

 ternal or dorsal (on the convex side of the curved shells). 111. 

 Gen. Ammonites, Ceratites, Baculites, Turrilites, Scaphites, Ancy- 

 loceras. 



CHAPTER LII. 



DISTRIBUTION OF THE MOLLUSCA PROPER 

 IN TIME. 



REMAINS of the Mollusca proper are found in greater or less 

 abundance in almost all the stratified rocks from the com- 

 mencement of the Silurian period up to the present day. 

 Speaking generally, the Tetrabranchiate Cephalopoda are the 

 chief representatives of the Mollusca in the Palaeozoic rocks, 

 the Lamellibranchiata and the Dibranchiate Cephalopoda in the 

 Mesozoic rocks, and the Gasteropoda in the Kainozoic period ; 

 but all the primary classes are represented even in the Lower 

 Silurian rocks. The following are the more noticeable facts 

 relating to the distribution of the various classes in past time. 



