The Fossil Grab of Gay Head. 589 



from Heteroplax Stm., (Prodr. dcscr. anim. evert, etc., 

 p. 40,) to which, of all existing Crustacea, our fossil genus 

 approaches nearest, it is removed by the presence -of a 

 median frontal emargination, and also, if I have rightly 

 interpreted the scanty material at hand for this point, in 

 the want of the antennary notch which separates the front 

 from the inner angle of the orbit. 



No fossil crab, to which Archaeoplax has any close rela- 

 tions has yet been described, with the exception of Arises 

 of De Haan, (Fauna Japonica, Crust., p. 62, PL v., f. 4,) 

 which differs from our genus in its shorter carapax, un- 

 armed antero-lateral margin, and narrower exognath of the 

 outer maxillipeds. Nothing is known with regard to the 

 formation in which Arges occurs. 



The study of the Archaeoplax, as an isolated fact, affords 

 but little aid in the determination of the age of the Gay 

 Head deposit. It belongs to one of the highest grades of 

 Crustacea, but still higher forms have been found in the 

 Eocene Tertiary, for instance Xanthopsis of McCoy in 

 the London clay. I was probably wrong in referring the 

 green-sand layer to the Cretaceous in the Am. Journal of 

 Science for January, 1860, although Dr. Hitchcock sug- 

 gests that this layer may consist of the debris of some 

 older deposit, such as the Cretaceous ; but there is noth- 

 ing to confirm this view in the character of the fossils 

 accompanying the crabs. 



The abundance of specimens of the Archaeoplax at Gay 

 Head, indicates a warm climate in that region at the time 

 they were living. At the present day all Carcinoplacidae 

 are inhabitants of warm seas. The nearest allied genus, 

 Heteroplax, hves on the coast of China at the northern 

 limit of the torrid zone. 



