HISTORICAL GEOLOGY 



The Tetrabranch Cephalopods, for two reasons, constitute one 

 of the most interesting and instructive illustrations of evolutionary 

 changes, ranging from the early Paleozoic to the present time, first 

 because we have such an abundant record in the rocks of all these 

 periods, and second because the evolutionary changes have ex- 

 pressed themselves in the external or shell portions in a remarkable 

 and easily recognizable manner. The only known Cambrian 

 Tetrabranchs were of the very simple, straight, or curved cham- 



bered-shelled types like the Orthoceras 

 and Cyrtoceras. In the Ordovician 

 the straight form, e.g. Orthoceras (Fig. 

 55a) was still dominant, but many ad- 

 vances were made giving rise to more 

 curved forms (e.g. Cyrtoceras, Fig. 

 55b), open-coiled forms (e.g. Trocho- 

 ceras, Fig. 55c), and close-coiled forms 

 (e.g. Trocholites, Fig. 55d) . All of these 

 forms belonged to the Nautiloid divi- 

 sion of the Tetrabranchs, that is, their 

 septa or chamber partitions, where in 

 contact with the walls of the shell, were 

 straight or at least very simple. Close- 

 coiled Nautiloids of the Ordovician 

 greatly resembled the modern Pearly 

 Nautilus, which is one of the very few 



SjMppiiiii^fi living representatives of the now al- 



BS8IP most extinct Nautiloids (Fig. 13). The 



1 v'' ,,, "TnlBi2iiS persistence of these simple close-coiled 



S'™IS ™ DI * ,: forms from the Ordovician to the pres- 



ent is noteworthy. Ammonoids, that 

 is to say Tetrabranchs with more com- 

 plex septa junctions, appeared in the 

 Devonian and became increasingly 

 prominent well into the Mesozoic era, 

 but they have not continued to the 

 present. 

 Since the Tetrabranchs are of such special interest from the 

 standpoint of evolution, the following tabular summary is given to 

 more clearly bring out certain prominent changes of shell structure 

 from Cambrian time to the present. 



Fig. 56 



An Orthoceras restored. (Af- 

 ter Nicholson, from Le 

 Conte's "Geology," cour- 

 tesy of D. Appleton and 

 Company.) 



