Manchester Memoirs, Vol. Iv. (191 1), No. 8. 13 



types to smooth forms. Neumayr (Neumayr 18S9), in his 

 paper on Trigonia, says that all Unionid ornament is post- 

 Miocene, and therefore atavistic. That can hardly be, 

 since the Laramie Unionids are fully ornamented. He 

 also states that the European forms are entirely unorna- 

 mented. In this, he leaves out of account the umbona 

 markings. 



The ornament in foreign forms is of three types : — 

 {a) Forms in which it is confined to the nepionic 



stages (as in British forms). 

 iji) Forms in which it occurs on the adult shell, and 

 is either phylo-ephebic or phylo-gerontic in 

 character. 

 ■ {c) Forms in which a phylo-gerontic smooth stage is 

 followed by irregular ornament, which may there- 

 fore be regarded as phylo-hyostrophic in character. 



(rt) The umbonal markings show three, and probably, 

 four types of ornament ; 



(i) The W-shaped ridges, shown by many N. 

 American, S. African, and European forms 

 (possibly excepting the Margaritanas). 



(2) Single V-shaped ridges, seen in some S. and N. 



American forms, U. nyassensis, and a form 

 from Madras. 



(3) Ridges which run diagonally up to the gonal line 



of the young shell. 



(4) Possibly a type from the Plate River and 



Guiana. Here the ridges appear to be purely 

 radial, but I am not certain whether they are 

 purely radial or only a variation of the V type, 

 which can approximate a radial type fairly 

 closely. 



