3i8 RUDOLF RUEDEMANN 



and considered as possible ancestors of Apus. Pompeckj holds 

 that neither this nor the leptostracan nature of these fossils can 

 be proved. We believe that the two muscle-impressions which 

 Ribeiria shows on the dorsal median-line^ indicate that in this form 

 the carapace was attached to the body in nearly its entire length 

 along the dorsal line while in Apus it is free from the cervical fold 

 backward, i.e., in its greater portion. This would already indicate 

 a very different organism, even if the deep transversal sulcus in 

 front of the apex of the casts of Ribeiria could in some way be 

 compared with the cervical fold in the carapace of Apus. 



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^Ribeiria compressa Whitfield, Bull. Amer. Mus. Nat. Hist. N.Y., Vol. I, No. 8 

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