METAZOA MOLLUSCA. 255 



their development passed through the loosely coiled, 

 tightly coiled, and involute stages. The Nautilus was 

 seen to be an involute form, but no reduced stage with 

 a more or less uncoiled spire was described since no such 

 form has been discovered. 



Among the Ammonoids, however, a number of series 

 have been traced from the straight primitive form through 

 the involute stages to the straight reduced condition. 



The Ammonites culminated in the Jurassic period. 

 At the close of this period in all probability there was 

 some great climatic change which caused reduction. The 

 Nautiloids were somewhat affected, but not enough to 

 cause them to become extinct. Those that died, did so 

 slowly. The Ammonites, on the contrary, developed 

 extraordinary reduced forms and afterwards became ex- 

 tinct. 



The first indications of reduction are a lateral contrac- 

 tion in the whorl (see No. 617, Sphaeroceras brochi Sow.), 

 attended with a diminution in the size and a decrease in 

 the vertical height (No. 618, Sphaeroceras wrighti}. In 

 some cases the process is carried so far that the terminal 

 portion of the last whorl separates from the preceding 

 whorls and grows out straight, as seen in Scaphites nodo- 

 sus Meek. The young Scaphites (No. 619, specimen on 

 the left, and the section, No. 620) are closely coiled, but 

 the older stage (No. 619, specimen on the right) has 

 begun to uncoil. This is still better seen in No. 621, 

 which is a cast of an aged specimen of Eurystomites kel- 

 loggi showing the free volution or whorl. 



Another old age form is seen in No. 622, Helicoceras 

 stevensoni Whitfield, where the asymmetrical spiral has 

 partly uncoiled, and the last whorl has a secondary back- 

 ward crook bringing the aperture of the shell near the 

 base of the spire. 



The uncoiled condition is carried still farther in Crio- 

 ceras bifurcatus Quenst. (No. 623), in which the shape 

 is more like a hook than a spiral. 



