554 



THE CRETACEOUS PERIOD 



Fig. 469. CRETACEOUS FOSSILS, a-e, Echinoderms: a, Pedinopsis pondi 

 Clark; b, Cassidulus subquadratus Con.; c, Botriopygus alabamensis Clark; d and e, 

 Salenia tumidiila Clark. /, g, and h, Pelecypods: /, Ostrea soleniscus Meek; g, 

 Idonearca nebrasccmis Owen, allied to the areas of to-day; h, Inoceramus vanuxemi 

 M. and H. i-l, Gastropods: i, Neptunclla intertextus (M. and H.);./, Aphorrhais 

 prolabiata (White); k, Drepanochilus nebrascensis (E. and S.); /, Pyropsis bairdi (M. 



and H.) 



h 



Fig. 470. CRETACEOUS CEPHALOPODS: a, Nautilus meekanus Whitf., one of the 

 simplest types of closely coiled cephalopods; b, Helicoceras stephensoni Whitf., an 

 ammonite coiled in a heliciform spiral, and c, its highly complicated suture; d, 

 Prionotropis woolgari (Mantell), a normal ammonite with ornamented shell, and e, 

 complex sutures;/, Ptychoceras crassum Whitf., an ammonite shell which is recurved 

 upon itself, but not coiled; g, suture of/; h, Scaphites nodosus Owen, an ammonite 

 showing as light tendency to uncoil in the last volution; i, Baculites grandis M. and H. 



Their biconcave vertebrae and other skeletal features, as well as 

 their small brains, suggest reptilian relationships. Their habitat 

 was the same as that of Hesperornis, and yet the two were farther 

 apart, structurally, than any two types of birds now living (Marsh). 



