from the Lower Helderberg Group. 145 



cyclurus Hall, nor to P. maerobhcs Billings, sp., as it has a 

 strongly denticulate posterior border, a character not possessed 

 by either of the described species. The generic reference can 

 be determined with more certainty, for the combination of 

 features in the cephalic and pygidial regions cannot well exist 

 in any genus except Phaethonides found associated with this 

 larva. 



The outline is broadly elliptical transversely, and obtusely 

 angular at the genal extremities. The cephalic portion is semi- 

 elliptical, and includes more than half the animal. It has a 

 narrow, flattened, entire border, and is defined posteriorly by 

 the obtuse points, which are correlated with the genal angles. 

 Palpebral lobes prominent, and, as in the larva of Aoidaspis, 

 situated nearer to the anterior margin than to the frontal lobe 

 of the axis. 



The pygidial portion is depressed convex, with five pairs of 

 symmetrically disposed marginal denticulations or spines, of 

 which the median pair is short. 



Axis narrow, extending nearly the entire length of the ani- 

 mal, obscurely annulated by six or seven (possibly eight) slight 

 constrictions. The anterior five seem to pertain to the cepha- 

 lon and the others to the pygidium. The annulations each 

 carry a single pair of spines, with the exception of one at the 

 junction of the cephalon and pygidium, which apparently has 

 but a single spine. 



The pleural and lateral facial areas also bear double rows of 

 spines, the anterior pairs being between the eyes and axis. 

 The distinction between the cephalon and pygidium is very 

 obscure, but the apparent large size of the latter in Phaetho- 

 nides may indicate a moult later than the first. The ventral 

 side, figure 9, shows a narrow doublure and the axial cavity. 

 Greatest length, l'15 mm ; greatest width, L34: mm . 



Observations. 



Many correlations have been drawn between the Trilobita 

 and other Arthropoda, by Packard, Dohrn, Lang, and others. 

 The present state of opinion is well expressed by Lang,* who 

 concludes that the Trilobites are related to the Crustacea. In 

 support of this is " the fact that the trunk feet are biramose 

 and carry epipodial appendages." The important discovery 

 by Mr. W. S. Valiant, of organs which can be homologized 

 with antennae, as announced by Mr. W. D. Matthew, f estab- 



* Text Book of Comparative Anatomy, English edition, p. 415, 1891. 



f On Antennas and other Appendages of Triarthrus Beckii, by W. D. Matthew, 

 read before the N. Y. Acad, of Sci., May, 1893. The first specimen preserving 

 the antennas was discovered by Mr. W. S. Yaliant several years since. Recently 

 Mr. Valiant and Mr. M. Sid. Mitchell have obtained other specimens showing 

 more completely the nature of the appendages. 



