«/. Starkie Gardner— 1 On Cretaceous Aporrkaidce. 395 





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Ornithopus 















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globulata, Seeley 











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X 



X? 



Moreausiana, cTOrb. ... 



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oligochila, Gard 













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pachysoma, Gard 













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Tridactylt/s 















cingulata, P . & R. ... 









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Griffithsii, Gard 









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Aporrhai's, Group I. 

















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X 



















X 



subtuberculata, Gard 











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ParMnsoni, Mant 





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Cunningtoni, Gard. 











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X 





Aporrhais, Group II. 















carinella, P. & C 









X 















X 















X 















X 















X 







DlMORPHOSOMA 















ancylochila, Gard 



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X 









doratochila, Gard 









X 





X? 



Jcinclispira, Gard 



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X 









opeatochila, Gard 













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plenrospira, Gard 



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spathochila, Gard. 













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Their mode of growth seems to me to have been as follows, which 

 is entirely different from that of any known recent shell. The 

 animal had the power of absorbing the upper or dorsal layers of the 

 wing at the same time that the ventral layers were deposited, in the 

 same manner that, the cowry removes the internal layers of its shell 

 wall, and deposits new layers externally with its overlapping mantle. 

 In support of the idea that the wing follows the aperture of the 

 shell, and is continuously absorbed behind by the left edge of the 

 mantle, I have seen in many of the specimens in Mr. Meyer's collec- 

 tion, that when small, oysters have grown on the back of the shell, so 

 as to interfere with the possibility of the absorption ; the old wing 

 has been simply left behind, and an entirely new one developed. The 

 figures which illustrate this are PI. XII. Figs. 12, 12a, 126. The ribs 

 were deposited after the formation of the whorl, by the inner margin of 



