606 MOLLUSCA 



and numerous fossil species, grouped in 2 suborders. Of these suborders, 

 the Ammonoidea, or the ammonites, were abundant throughout the Meso- 

 zoic age and became extinct before its close. 



SUBORDER NAUTILOIDEA. 



Shell straight (Orthoceras), bent (Cyrtoceras), loosely (Gyroceras), 

 or closely coiled (Nautilus), in the fossil species, and closely coiled in all 

 the living species; septa concave; siphuncle variable in position, often 

 large: 11 families, with 4 living and 2,000 fossil species, the latter dating 

 back to the Cambrian and reaching their maximum in the Silurian and 

 Devonian ages. 



FAMILY NAUTILIDAE. 



Shell with few whorls, more or less overlapping; septa simple; 

 siphuncle central or nearly so ; tentacles about 90 in number, consisting of 

 4 groups of 12 or 13 labial tentacles around the mouth, 2 groups of 17 

 larger tentacles on each side of the head, 2 thicker tentacles, which combine 

 to form a hood beneath which the mouth and 

 its appendages can be retracted, and 2 small 

 tentacles on each side of the eye: 1 genus. 



NAUTILUS L. With the characters of the 

 family: 4 species; 300 fossil species. 



N. pompilius L.* The pearly nautilus 

 Fig. 952^ction of a && 952 >- She11 U P to 25 cm * in Diameter, 

 Naturaf mstory) Cambridge white with chestnut-brown stripes, and pearly 

 inside; body about 18 cm. long: Pacific and 



Indian Oceans, in rather deep water near the shores; often common; 

 body used for food, and the shell extensively employed for ornamental 

 and useful purposes. 



ORDER 2. DIBEANCHIATA. 



Shell internal (except in Spirula) or absent, either homy or cal- 

 careous, and not coiled (except in Spirula) ; 2 ctenidia, 2 kidneys, and 2 

 branchial hearts present; siphon tubular; mouth surrounded by either 

 8 or 10 arms, which have suckers or hooks; ink sac present; eye with a 

 crystalline lens ; body cylindrical or globose : about 350 species, grouped in 

 2 suborders; about 30 species off the Atlantic coast. 



Key to the suborders of Dibranchiata : 



<*! Ten arms present 1. DECAPODA 



a, Eight arms present 2. OCTOPODA 



* See "The Anatomy of Nautilus pompilius," by L. E. Griffin, Mem. Nat. A cad., 

 Vol. 8, p. 103, 1900. "Notes on Living Nautilus," by Bashford Dean, Am. Nat., 

 Vol. 35, p. 819, 1901. 



