TNOCERAMTTS. 



333 



and a relatively short hinge are still retained, and the right valve is still convex 

 and has a concave anterior area. A variety of /. vivolufiifi links snch intermediate 

 forms with typical examples of involidus ; in that variety the right valve is rather 

 more convex than in typicnl forms, the left valve is not so distinctly spiral, some 

 trace of the anterior flattened or concave area is still retained, and the hinge-line 

 is rather shorter relativelv. 



-- «;jv:^pxEmq 



Fig. 91. — /iiorcramus into(«(ns, Sow. Upper figure : Zone of iVic)"as(error-niigHiiir(«i. Gravesond. Sedgwick 

 Museum, Cambridge. Kiglit valve witli marginal growth round the hinge, x J. 

 Lower figure : Hinge of right valve ; Upper Chalk, Norfolk. Norwich Museum, No. 3355. The anterior 

 part of the hinge is partly concealed by the marginal growth of the shell, x |. 



/. nmhnnatns, Meek and Hayden,' fiom Fort Benton, Missouri, is, as stated by 

 Meek, very closely allied to, and perhaps identical witii, /. incolidHS. Another 

 related form is /. exogyroides. Meek and Hayden.'^ Both are regarded as synonyms 



1 'Invert. Cret. and Tert. Foss. U. Missouri' (1876), ]>. 44, pi. iii, fi<.'. 1 : jjI. iv. tigs. 1, 2. 



2 Ibid., p. 46, pi. V, fig. 3. 



44 



