TRILOBITA. 365 



behind them, were also biramous appears to rest on insecure 

 foundation. 



The Utica shales have also yielded specimens of Trinudeus 

 in which the pygidial appendages are displayed, and these 

 resemble the corresponding structures in Triarthrus, though 

 considerably shorter. 



Before the discovery of this material much light was thrown on the nature 

 of the appendages of Trilobites by Walcott who examined a large number 

 of fossils of Calymene by means of sections. He concluded that two long 

 and sometimes spirally twisted epipodial lobes were appended to the 

 outer side of the base of the biramous limb. In view of the difficulty of 

 arriving at certain results by this method, and of the fact that Triarthrus 

 yields no trace of such structures, their existence in Calymene appears 

 very doubtful. 



Development. Remains of young trilo- 

 bites are sometimes found associated with 

 those of adults, and the successive stages of 

 the development of the head and trunk 

 have been traced by Barrande and by 

 Beecher. In the earliest which has been 

 recognized, the protaspis stage of the latter FIG. 246. Larva of 



manites sociahs Bar. 



author, they are minute oval bodies '4-1 mm. in . the "ana-protas- 



pis " stage. A pygi- 



in length, with indications of the five divi- dium of three segments 



is present, but there 



sions of the glabella, and of one succeeding are as yet no free 



thoracic segments. 



segment. The eyes are anterior and mar- x so. From Beecher, 



after Barrande. 



ginal when they first appear, the subse- 

 quent change in position causing the indentation of the ocular 

 suture of the adult. The appendages of the young have not 

 been seen. 



The fact, pointed out by Barrande, that in Trinudeus, and other genera 

 a pygidium is formed before the number of free thoracic segments is 

 complete, does not, as might at first appear, necessarily show that the 

 region of the formation of fresh segments was in front of the pygidium. 

 We are at liberty to suppose that the formation of fresh segments occurred 

 at the posterior end of the body, as in most other young Crustacea, and 

 that segments which at one stage of growth belonged to the pygidium 

 became in a later stage, after casting a shell, free thoracic segments. 



Occurrence. The remains of Trilobites are associated in 

 marine strata with those of Crinoids, Brachiopods and Cephalo- 

 pods. The absence of eyes in some cases is perhaps an indica- 

 tion that the species so characterized inhabited deep water. 



In view of the high interest attaching to Trilobites as ancient 



