PYGOCEPHALUS COOPERI 469 



gradually increase in width from before backwards ; so also, on this 

 reading of the fossil, do those of Pygocephalus. 



The abdomen of Pygocephalus, however, is much thicker and 

 stronger in proportion than that of Mysis ; and its telson and the 

 appendages of the last somite, which together constitute the caudal 

 fin, differ greatly in form from those of Mysis, being far wider. The 

 outer edge of the caudal fin again in Mysis is nearly straight, while in 

 Pygocephalus it is much curved. 



In all these respects Pygocephalus more nearly approximates to 

 the SquillidcB ; and I have given a sketch of a Gonodactylus bent 

 upon itself, and viewed from the ventral side (fig. 6), as I suppose 

 the fossil to be, in order to show how closely the general proportions 

 of the two genera approximate. 



I cannot imagine that all these coincidences are accidental, and I 

 conclude therefore that Pygocephalus is a Podophthalmous Crusta- 

 cean in all probability more nearly allied to Mysis than to any other 

 existing form. 



At any rate we shall be quite safe in assigning to it a position 

 among either the lower Decapoda or the Stomapoda ^ ; and supposing 

 the interpretation which I have given of this difficult fossil to be well 

 founded, it affords, so far as I am aware, the first certain evidence of 

 the existence of Podophthalniia at so early a period as the Carboni- 

 ferous epoch.^ 



' I have elsewhere (Lectures on General Natural History, ' Med. Times and Gazette,' 

 May 23rd, 1857) given my reasons for limiting Xht gxa\i\> oi Stomapoda \.o Squilla wcvA its 

 immediate allies. The Schizopoda, including Mysis, are not essentially different from 

 Decapoda. 



^ If the genus Gitocrangoti, described by Richter (Beitrag zur Palaontologie des Thiiringer 

 Waldes, p. 43), really constitute, as its discoverer considers, a transition between \he Macrut-a 

 and Brachyura, it is not only an earlier, but a more highly organized Crustacean. 



