6O THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



within 0.5 to i mm. of the neck furrow. The posterior pair of appendifers are behind the 

 hypostoma, while the second pair are in front of its posterior end but escape being covered 

 by it on account of its oval shape. At the anterior end the hypostoma is widened by the 

 presence of two side lappets which extend beyond the boundaries of the glabella. In both 

 Ceraurus and Cheirurus the anterior edge of the hypostoma fits against the doublure at the 

 anterior margin of the head and the epistoma is either entirely absent or is so narrow as not 

 to be seen in specimens in the ordinary state of preservation. A section across the cephalon 

 of Ceraurus pleurexanthemus at the horizon of the eyes shows the sides of the hypostoma 

 fitting closely against the sides of the glabella (Walcott's pi. i, fig. i). Further back on 

 the head it is not in contact with the dorsal test, and the gnathobases extend beneath it. 



Restoration of Ceraurus pleurexanthemus. 

 (PI. ii ; text fig. 19.) 



The restoration of the appendages of Ceraurus pleurexanthemus is a tentative one, based 

 upon a careful study of the translucent sections prepared by Doctor Walcott. In no case 



Fig. 19. Restoration of a transverse section of the thorax 

 of Ceraurus pleurexanthemus Green, showing the relation of the 

 appendages to the appendifers and the ventral membrane. The 

 probable positions of the heart and alimentary canal are 

 indicated. 



among these sections is the actual test of any appendage preserved, and the real form of each 

 part is generally obscured by the crystallization of the calcite which fills the spaces formerly 

 occupied by animal matter. 



No section shows anything which can be identified as any part of the antennules, so 

 that these organs have been supplied from analogy with Triarthrus. 



There are undoubtedly four pairs of biramous Cephalic appendages, but their points of 

 attachment are not so obvious. There are two pairs of conspicuous appendifers on the 

 posterior part of the cephalon and another pair almost concealed by the hypostoma. It is 

 probable that the appendages of the cephalon were not attached directly beneath them, as 

 the four pairs have to be placed within the space occupied by the three pairs of appendifers. 

 As the mouth is in front of the posterior end of the hypostoma, the gnathites of the first 

 pair of biramous appendages may have extended beneath that organ, or they may have lain 

 beside it, and only become functional when the hypostoma was dropped down in the feed- 

 ing position. The second pair of gnathites reached just to the tip of the hypostoma, and the 

 other two pairs seemingly curved backward behind it. 



The points of attachment on the thorax, as shown clearly in sections, were directly be- 

 neath the lower ends of the appendifers. The endopoclites were long enough to reach to 

 or a little beyond the outer extremities of the pleural spines, while the exopodites were 

 apparently somewhat shorter. Each endopodite consisted of six short, fairly stout seg- 

 ments, each with at least two spines on the somewhat expanded distal ends. The exact 



