Vol. 50.] SYSTEMATIC POSITION OF THE TEILOBTTES. 



423 



Apus its exact position with reference to the segmentation is difficult 

 to ascertain. As some evidence of this wandering backwards of the 

 organ in the trilobites, I would draw attention to the sloping back- 

 wards of the lines of constriction between the posterior head-segments 

 shown in fig. 9 ; and further to the fact that Walcott describes a 

 tubercle on the fourth (last) head-segment of Microdiscus, whereas, 

 where five segments form the head, it is generally found on the fifth. 

 Asaphus seems to form an exception, for the tubercle (?) appears to 

 occur on the fourth segment, and not on the fifth. 



Fig. 9. — Head-shield of Olenellus (Mesonacis) asaphoides, showing 

 the oval '■dorsal organ' on the fifth cephalic segment. 



[From Tenth Eeport U.S. Geol. Surv. (1890) pi. xc] 



This organ seems, in the trilobites as in the Crustacea, to have 

 been very early modified. It develops in the former into a slightly 

 conical prominence in Isotelus, or into a long sharp spine, e. g. in 

 Olenellus Broggeri. Traces of it appear in very many Cambrian and 

 Silurian trilobites, for example, in species of Dalmanites, Asaphus (on 

 the fourth segment), Cheirurus, Bronteus, Proetus, Cyphaspis, Acid- 

 aspis (either as a median spine or as a circumvallate pit between 

 two lateral spines), Conocephalites, Hydrocephalus. In the Carbo- 

 niferous trilobites figured in Dr. Woodward's monograph, 1 traces of 

 it are marked in species of Phillipsia and Grijjithides. In many of 

 these it occurs as a round mark, the exact nature of which is difficult 

 to ascertain. In Olenellus Broggeri and in some species of Sao and 

 Acidaspis, as above stated, it is produced into a sharp median spine. 

 That all these structures are modifications of the oval patch on the 



1 Palaeont. Soc. vols, xxxvii. & xxxviii. 



