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apical foramen, usually closed at an early age. Hinge area flat, 

 triangular ; deltidium triangular, hardly distinguishable from 

 the area ; interior deeply concave, furnished with two prominent 

 cardinal teeth. Brachia often irregular, separated by a median 

 ridge from which other ridges branch out, often unsymmetrically. 

 In the excavations between these ridges the brachia are placed 

 like a fringe upon the mantle. 



Hoemal valve furnished with a prominent cardinal process be- 

 tween the sockets, and this process is frequently broadly chan- 

 nelled. Just before it is the cavity for the viscera, which is often 

 overshadowed by the calcareous network which supports the 

 mantle. 



Fig. 25. Fig. 26. 



Fig. 25. Thecidium radians, neural valve : /, rudimentary foramen ; d. delti- 

 dium ; A, hinge area; a, cavity for adductor ; jo, dental sockets, y nat. size. 

 Fig. 26. HEemal valve of the same, f nat size. 



Type. Thecidium pumilum, Val., apud Lam. sp. 1819 ; Hist. 



Nat. p. 58. Dav., An. Nat. Hist. 1850, pi. xiv, fig. 5S. 

 = Thecidea radiata, Deh-., Fer. Tab. Syst. 38, 1821,+ T. 



papillata, Bronn. 

 Fossil, cretaceous beds of Europe. 



After a careful study of the admirable plates of M. Deslong- 

 champs, with the paper of M. Suess which accompanies them, as 

 well as of the observations on the genus by Messrs. Davidson, 

 Deslongchamps and others, I am compelled to dissent entirely 

 from the views of those naturalists in regard to the homologies 

 of the internal calcareous network which is so remarkable in 

 Thecidium. 



I can see no grounds for considering this network as equiva- 

 lent to the loop of Megathyris or any other genus of the Terebra- 



