Percy E. Raymond— The Pygidium of the Trilohite. 23 



close relationsliij) of D. eucentrus and D. mucronatus. The latter 

 species appears first in the Slaurocejohalus beds, which are beneath 

 the Brachiopod Shales containing D. eucentrus. The pygidium of 

 the adult D. mucroyiatus is larger than that of the other species, 

 having eight joairs of pleural lobes. In short, D. eucentrus seems to 

 be descended from D. mucronatus, and in its youth passes through 

 a stage in which it has a large pygidium like the latter. 



All of this shows that the free segments of the thorax become 

 such by the breaking down of a large pygidium, and that a small 

 pygidium is the result of the degeneration of a large one. Further, 

 that a large pygidium is not made up by the fusion of segments 

 once free, and so is not a specialized organ in the sense that the 

 metasomic shield of Limulus is. 



If trilobites be examined with the assistance of these facts, rather 

 than with the aid of the theory of origin from a numerously segmented 

 annelid, much of the supposed specialization of Hypoparian trilobites 

 disappears. The Agnostidae, Eodiscidse, Trinucleidse, and the like 

 are doubtless specialized — nothing which is not specialized can 

 exist — but they are speciahzed simple trilobites. This is shown 

 not only by the small number of free segments and relatively large 

 pygidia, but by the absence of comjDound eyes and the presence of 

 marginal or submarginal facial sutures, features in which they agree 

 with the simjDlest type of protaspis. 



The Pygidium in the Agnostid.e. 



As to which shield is which in the Agnostidae, I can only say 

 that I have exhausted the collections accessible to me in searching 

 for a specimen retaining the hypostoma. I have brought the 

 case to the attention of Doctor Walcott and his assistants, and 

 Mr. B, F. Howell, of Princeton University, is going to make 

 a special search in the shale of the Middle Cambrian of South-Eastern 

 Newfoundland this summer. I appeal to any student having 

 specimens j)reserved in shale to make an attempt to find a specimen 

 with the hypostoma in position. There are other possible lines of 

 evidence, but none so satisfactory. Pending such evidence it may 

 be pointed out that no other trilobite has a free plate on the ventral 

 side of the pygidium, that Wahlenberg, Dalman, Brown, and 

 Hisinger used an orientation the reverse of that now accepted, and 

 that as now placed the tips of the pleurae of the thoracic segments 

 curve forward instead of backward as in other trilobites. If one 

 take plate vi of Angelin's Pal(Bontologia Scandinavica and reverse it, 

 he will find that all of the figures appear more natural. In the 

 accepted position the tips of the pleurae of the thoracic segments 

 and the pleural furrows point forward. In the reversed position 

 they point backward, as in all other trilobites. In the accepted 

 position every species has a tubercle on the median line of the 

 pygidium. In the reversed position this would be on the cephalon, 

 where it would more naturally occur. In the accepted position. 



