348 



ANNUAL EEPOBT SMITHSONIAN INSTITUTION, 1916. 



can boast of a long line of progenitors ; indeed, there are few groups 

 that can compare with them in this respect. For millions upon mil- 

 lions of years ago, or to be more precise, in Upper Cambrian times, 

 there existed a small nautiloid animal in the seas, whose deposits are 

 known as the Chau-mi-tien limestone near Tsi-nan, Shantung, China. 

 The shell of this little animal, which was christened C yrtoceras cain- 

 hria by Dr. Walcott in 1905,^ is only 7 millimeters in length and 3 

 millimeters in diameter (fig. 1). 



Ever since that time, and probably long before this tiny, flexed, but 

 noncoiled chambered nautiloid ancestor of the Cephalopoda existed, 

 chambered nautili were living somewhere in our 

 seas. The Ozarkian period ushered in a number 

 of families, each with its genera and species. 

 The Canadian added materially to these, but the 

 greatest differentiation of all took place in the 

 Ordovician and Silurian, after wdnch the decline 

 of the order began, resulting finally in the rem- 

 nant of four closely allied species belonging to 

 the single now existing genus Nautilus. In all, 

 about 3,000 species have been named and to their 

 number new^ forms are constantly being added b_v 

 the patient paleontologist. In all these forms we 

 have the shell divided into chambers by trans- 

 verse concave septa whose margins may be 

 straight or undulate ; a siphuncle or tube extends 

 from chamber to chamber coamecting them with 

 each other. The range of variation in shape and 

 size is quite great. There are straight cones, as 

 in Orthoceras; flexed forms, as in Cyrtoceras; 

 looseh' coiled forms, as in Sphyradoceras; closely coiled forms, as in 

 Nautilus; or even closely coiled and finally solute shells, as in 

 Ophidioceras and Lituites. The sculpture, too, j)resents no end of 

 variations, for some shells are smooth, others axially or spirally 

 striate, or channeled; or Urate, or threaded, ribbed, or keeled, or 

 marked by combinations of these elements, some even have tubercles 

 and bosses, but whatever the sculpture or size, which varies from the 

 7-millimeter ancestor to the 14-foot or more long cones of Endoceras, 

 one word characterizes the entire group, and that is elegance (pi. 1). 

 During the Uppier Silurian period a new offshoot of the Cepha- 

 lopod stock developed, a stalk which has far excelled the Nautiloids 

 in numbers as well as in diversity of structure. We refer to the order 

 Ammonoidea, " the Ammon's horns," of which probably more than 



Fig. 1. — Cyrtoceras cam- 

 Iria Walcott. The an- 

 cestor of the Cephalo- 

 poda. Side view, X 5. 

 End view, X 7. 



iProc. U. S. Nat. Mus., Vol. 29, p. 22, 1905. 



