OLENELLUS AND OTHER GENERA OF MESONACID.Ti 23/ 



back against the palpebral ridge, and the latter, with the occular 

 segment, terminates against the large intergenal spine formed by the 

 prolongation of the glabellar segments. The pygidium is a simple 

 plate without axis or traces of segmentation. The young of Pcc- 

 dcumias transitans [pi. 25, fig. 22] of a little later stage of growth 

 lias the segmentation of the cephalon finely shown, also remarkably 

 long, intergenal spines. 



The progressive changes of the cephalon result in the gradual 

 separation of the intergenal and genal spines and the straightening 

 out of the posterior margin. This occurs in Pccdcuiiiias [pi. 25, 

 figs. 20-22], EUiptoccphala [pi. 25, figs. 9-12], and Wanncria [pi. 

 31, figs. 8, 7. 5, and 6]. A curious phase in the later development 

 of the cephalon is the advancing of the genal angles from the line 

 of the occipital segment until they are forward of the anterior mar- 

 gin of the glabella. [See pi. 32 for Pcrdemiiios, pi. 25 for EUipto- 

 ccphala, pi. 31 for Wajiiicria. and pi. 37 for OlcncUus.] 



The genal, intergenal, and antero-lateral spines of the cephalon 

 undoubtedly represent the pleural ends of segments that have been 

 fused together and greatly modified in the process. The genal 

 spines persist in the adult of the ]\Iesonacidas and often the inter- 

 genal spines, but only in a modified manner. The intergenal spines 

 are seen in a later geological period in the adult Brontciis^ where 

 they might be considered as a reversion to a character of their Cam- 

 brian ancestors. Hydrocephalus ' appears to have an intergenal spine 

 and in all of the Proparia [Beecher, 1897, p. 198] the "genal spine" 

 is attached to the space within the facial sutures, and is in fact the 

 prolongation of one of the fused segments of the cephalon, and 

 corresponds in this respect to the intergenal spine of the Mesonacid?e. 

 Some of the species of the genus Agnostus also show spines that 

 suggest the intergenal spine, notably A. grannlatns Barrande and 

 A. rex Barrande.^ 



Nufiibcr of segments in the cephalon. — The question of the num- 

 ber of segments represented in the cephalon is one that cannot be 

 fully discussed here.* The presence of a palpebral segment that 

 appears to also form the larger part of the anterior glabellar lobe 



' Barrande, 1872, pi. 9, figs. 12 and 13 ; and pi. 11, figs. 13 and 14. 



' Barrande, 1852, pi. 49. 



' Barrande, 1852, pi. 49. 



* The student is referred to a paper by H. J\I. Bernard on the " Systematic 

 Position of the Trilobites," Quart. Journ. Geo!. Soc, London, Vol. 50, 1894. 

 pp. 411-432; also to C. E. Beecher's paper on the " Larval Stages of Trilobites," 

 American Geologist, Vol. 16, 1895, pp. 166-197. 



