238 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



in the young of EUiptocephala [pi. 25, figs. 9 and 10] , Pccdeiimias 

 [pi. 25, fig. 22], and Olcnellns [pi. 36, figs. 11-14], and the posterior 

 portion of the same lobe in Olenellus logani [pi. 41, fig. 6], is evi- 

 dence that there are six, if not seven, clearly defined segments in 

 the cephalon ; these include the occipital ring, the four segments rep- 

 resented by the glabellar lobes, and the occular or eye-bearing seg- 

 ment ; the expansion of the latter may form the anterior portion 

 of the first glabellar lobe as indicated in Olenellus logani [pi. 41, 

 fig. 6] , where the furrows on the glabella in advance of the palpebral 

 segment apparently outline the segment. In all trilobites where the 

 cheeks carry the visual surface of the eye, the cheeks may be con- 

 sidered as an expansion of the occular segment, and probably of a 

 segment in advance of it, and the genal spines are the outward ter- 

 mination of the occular segment. The anterior and second segments 

 of the glabella do not appear to terminate in spines, but the third 

 or fourth segment may be extended into the intergenal spines [pi. 

 25, figs. 9, 10, and 22 ; pi. 39, fig. 6] . 



It is not improbable that a seventh segment more anterior than 

 the occular segment existed in the primitive cephalon of the Mes- 

 onacid^e ; this is indicated by the antero-lateral spines of the young 

 of Olenellus gilberti [pi. 36, figs. 11-14] and the larval-like cephalon 

 of Olcnelloides [pi. 40, figs. 2 and 3] and by the cephalon of Cal- 

 lavia hicensis [pi. 41, fig. 9] where there are two pairs of furrows 

 in front of the palpebral ridge. 



The preceding brief outline of the segments included in the ceph- 

 alon may be tabulated as follows : 



1. Anterior border segment, the reflected margin of which sup- 

 ports the hypostoma. 



2. Occular segment carrying the visual surface of the eye. 



3. Palpebral or first glabellar segment from which the large an- 

 terior lobe of the glabella was largely developed, also the so-called 

 " occular " ridge, and the palpebral lobe. 



4. Second glabellar segment which is usually extended beyond the 

 end of the third glabellar segment in the adult cephalon. 



5. Third glabellar segment which may or may not be extended so 

 as to appear in the interpalpebral space. 



6. Fourth glabellar segment. This segment in the young [pi. 25, 

 figs. 9, 10, 13, and 22; pi. 36, fig. 12] may be continued as an inter- 

 genal spine. 



7. Occipital segment, the extensions of which terminate against 

 the intergenal spines [pi. 25, figs. 9, 10, and 22]. 



