TNOCERAMUS. 



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 /. involutus links such intermediate 

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

 more convex than in typical 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 relatively. 



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Fig. 9i. — Inoceramus involutus, Sow. Upper figure : Zone of Micraster cor-anguinvm, Gravesend. Sedgwick 

 Museum, Cambridge. Right valve with 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 J. 



I. umbonatus, Meek and Hayden, 1 from Fort Benton, Missouri, is, as stated by 

 Meek, very closely allied to, and perhaps identical with, /. involutus. Another 

 related form is /. exogyroides, Meek and Hayden. 2 Both are regarded as synonyms 



1 'Invert. Cret. and Tert. Foss. U. Missouri' (1876), p. 44, pi. iii, fig. 1 ; pi. iv, figs. 1, 2. 



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



44 



