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AMERICAN JOURNAL 



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. 



Haemal 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. i 



Fig. 25. 



Fi?. 26. 



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

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

 Fig. 26. Haemal valve of the same, y nat size. 



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



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

 = Thecidea radiata, Defr., 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- 



