470 



Forty-fifth Report on the State Museum. 



general deposition about the margins of the aperture from 

 the apex downward. This plate has been termed the chi- 

 lidium (d') and has been shown to be of secondary growth. 



Variations in Form. 



The forms assumed by the brachiopods are so diverse that it 

 is often difficult to believe that shells so unlike in contour belong 

 to the same natural group. The habits of these animals and the 



Fig. 15.— MeristeUa nasuta. 



Fig. 16.— Pygope diphya. 

 p. Vascular sinuses. 



(Davidson.) 



Fig. 17. — Lingula paliformis. 



workings of natural causes in the modification of their shells 

 afford a very substantial aid to the comprehension of these 

 differences. 



The youngest shells of all Brachiopoda (so far as they have 

 been studied) are biconvex and sub-semicircular in outline. Those 



Fig. 18.— Ortitis subcarinata. 



shells in which the pedicle retains its functions until maturity 

 (Lingula, Terebratula, Magellania, Rhynchonella, etc.) and 

 hang freely suspended from the point of attachment, arc oval and 

 biconvex throughout their existence. In such shells as Lingula 



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