PYGOCEPHALUS COOPERI 467 



A glance at the different specimens which have been described is 

 sufficient to convince the investigator of their Crustacean nature ; 

 but in endeavouring to make a further step, and to determine the 

 precise order of Crustacea to which No. i was to be referred, I was 

 for a long time obstructed by the difficulty of deciding which end 

 was the head and which the tail, and whether the surface exposed to 

 view in this and the other specimens was the ventral or the dorsal. 

 At first I was inclined to regard the semicircular disk as a cephalic 

 buckler ; and, as the edges of this disk clearly overlap some of the 

 limbs, I was led to think that the dorsal surface was visible and 

 that I had before me a most anomalous form, with a head something 

 like that of an Apus or a Trilobite, the thorax of an Isopod, with 

 the limbs of a Schizopod, and with the abdomen and caudal append- 

 ages altogether peculiar and anomalous. I was inclined to imagine, 

 therefore, that this singular form combined the characters of several 

 orders of Crustacea. 



The high palseontological interest attaching to the discovery of such 

 a form, however, led me to give additional weight to every argu- 

 ment adverse to the validity of this provisional interpretation ; and, 

 apart from these considerations, there were several circumstances 

 which were great obstacles in the way of this view of the matter. 

 I could not understand the nature of the elongated plate attached to 

 ■one side of the quadrate disk in No. i ; the supposed caudal fila- 

 ments were wholly without parallel in the Crustacean series ; and I 

 could not see in what part of the body the segments exhibited in 

 No. 3 had their place. Furthermore, the form and apparent annu- 

 lation of the under surface of the supposed cephalic buckler were 

 quite incomprehensible. 



All these doubts were greatly strengthened when Mr. Cooper 

 kindly sent his specimen No. 2 for my inspection, which, exhibiting 

 a flat plate attached to both sides of the quadrate disk, clearly 

 proved that the one plate of the Manchester specimen was not a 

 merely adventitious appearance, while the deep excavation at the end 

 corresponding with the hemispherical disk seemed to show that, in 

 this specimen also, the corresponding division of the body was very 

 convex inferiorly. Now I know of no Crustacean possessing a 

 cephalic buckler, in which that region is more convex ventrally than 

 dorsally. 



Perplexed by all these doubts, I next reversed my hypothesis ; and, 

 assuming the quadrate disk to be the head, the hemispherical disk 

 to be the caudal extremity, and the exposed face to be ventral, — I 

 sought to ascertain whether, by working out the necessary conse- 



H H 2 



