282 MANUAL OF ZOOLOGY. 



Fam. 5. Sepiadce. 



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

 nating posteriorly in an imperfectly chambered apex (' phragma- 

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



Shell, internal, nacreous, chambered, discoidal; the whorls 

 separate ; a ventral siphuncle. Genus. Spirula. 

 ORDER II. TETRABRANCHIATA. 



Animal with four gills ; arms more than ten, without suckers ; 

 no ink-bag ; shell external, chambered, and siphuncled. 

 Fam. 1. Nautilida. 



Sutures of the shell simple ; the siphuncle central, subcentral, 

 or near the concavity of the curved shells, simple. 

 Sub-family Nautilidce proper. 



Body-chamber capacious ; aperture simple ; siphuncle 

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

 ceras. 

 Sub-family Orthoceratida. 



Shell straight, curved, or discoidal; body-chamber 

 small ; aperture contracted ; siphuncle complicated. 111. 

 Gen. Orthoceras, Phragmoceras, Cyrtoceras. 

 Fam. 2. Ammonitida. 



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 external 

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

 Ammonites, Ceratites, Baculites, Turrilitcs, Scaphites, Ancyloceras . 



CHAPTER LII. 



DISTRIBUTION OF THE MOLLUSC A 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. 

 Lamellibranchiata. The Lamellibranchs are known to have 

 existed in the Lower Silurian period, and have steadily in- 

 creased up to the present day, when the class appears to have 

 attained its maximum, both as regards numbers and as regards 

 variety of type. The recent bivalves are, also, superior in 

 organisation to those which have preceded them. Upon the 





