468 PVGOCEPHALUS COOPERI 



quences of this supposition, I should not arrive at a result more in 

 accordance with some of the known modifications of the Crustacean 

 plan. 



Upon this hypothesis, the small internal pair of appendages to the 

 quadrate disk are antennules ; the large external ones antennae ; the 

 seven segments are the sterna of seven thoracic somites, increasing 

 in width from before backwards ; the narrow longitudinal plates are 

 the edges of a short carapace ; and the semicircular disk is the ter- 

 mination of a large abdomen bent upon itself, and having its caudal 

 plates flattened out and crushed upon its anterior part. The twO' 

 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 (/') present two subcylindrical basal joints ; the 

 antenucE have two large basal joints giving attachment to a large 

 scale (y) externally and superiorly, while, internally, the fusiform 

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

 with 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 : sa 

 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 



