368 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [A})ril 22, 



and a half segments in No. 3 are, on this hypothesis, nothing but so 

 many of the abdominal somites viewed laterally. 



If I had been acquainted with no part of these specimens but the 

 quadrate disk and the segmented central portion of the body, I should 

 have had no hesitation whatsoever in adopting this view of their 

 nature ; and even although the assumption that the semicircular 

 disk is the crushed terminal portion of the abdomen may seem some- 

 what bold, yet it is the sole obstacle in the way of the only hypo- 

 thesis which enables us to bring this singular form within the cate- 

 gory of ordinary Crustaceans. 



For if, adopting this theory of the sides and ends of the fossil, we 

 compare it with the little Mysis or " Opossum-shrimp " of our own 

 seas, we shall find some curious points of resemblance between the 

 two. 



In Mysis (fig. 5), as in Pygocephalus, the abdomen is very large 

 as compared with the thorax, and the carapace is short and delicate. 

 The antennules (i') present two subcylindrical basal joints ; the 

 antennae have two large basal joints giving attachment to a large 

 scale (2") externally and superiorly, while, internally, the fusiform 

 base of the internal division of the antennae is formed by three joints, 

 vnth the last of which a very long multiarticulate filament is con- 

 tinuous. 



There are seven pairs of conspicuous thoracic members in Mysis, 

 the first pair of thoracic appendages (last cephalic of Milne-Edwards) 

 being smaller than the others and applied against the mouth ; so 

 there are seven pairs of appendages in Pygocephalus, but the nature 

 of the oral appendages in the fossil does not appear. 



In Mysis again the thoracic limbs (fig. 4) are short and feeble, 

 and consist of two parts, an endopodite and an exopodite, the latter 

 being terminated by a many-jointed filament. They present the 

 same peculiarities in Pygocephalus. 



In Mysis the sterna of the thoracic somites are well developed and 

 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 1 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 (piite safe in assigning to it a position 



