Report <>t the State GEOLOGIST. 



469 



the pedicle; in the former the foramen is usually apical, but in 

 the latter it generally lies below the apex, and in both oasesil may 

 become entirely obliterated by the later growth of the shell. 

 Likewise both deltidium and deltidial plates may be resorbed in 

 the senile stages of growth. 



In some of the edentulous genera, as Okbiculoidea and 

 Schizotreta, a structure somewhat analogous to the deltidium is 



B 



Pedicle valve. 



Figs. 13.— Orbiculoidea. 

 Profile. 



Brachial valve. 



V. pedicle- valve; D. brachial valve; o. apex of brachial valve; a apex of pedicle valve; 

 /. foramen; It. listrium. 



formed between the apex of the pedicle-valve and the opening 

 for the pedicle, and this area becomes modified by the passage 

 of the pedicle over its surface. It is important to distinguish 

 this from the deltidium of the articulate genera, and it may be 

 termed a listriate deltidium or listrium (It). 



It is probably true that in the very early growth stages of 

 all brachiopods, the posterior margin of loth r -\ 

 valves was grooved for the passage of the ^^^ 

 pedicle. This is to some extent the condi- 

 tion in the adult Linoula, Obolus and 

 Obolella. With the reduction in the rela- 

 tive size of the pedicle, and its restriction 

 to the larger valve, the primitive aperture 

 in the brachial valve was closed by a trans- 

 verse plate similar in appearance to the u — r m in. 

 deltidium, and in function (to close a dis- a . Apex of pedicle valve: 

 carded or abbreviated passage) to the deltidial JK P edicle - 

 plates of the opposite valve. This plate was not formed by 

 the secretion and union of distinct lateral plates but by a 



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