Vol. 50.] SYSTEMATIC POSITION OF THE TRILOBITES. 



425 



(fig. 11). These antennae, as far as can be ascertained, were attached 

 on each side of the labrum, 1 and may be assumed to have belonged 

 to the first segment, that is, they were homologous with the first 

 antennae of Apus. These very pronounced antennae were evidently 

 specialized in this particular trilobite ; but we may naturally infer 

 from them that all trilobites had appendages on the first segment 

 which were, as a rule, sensory organs. The exact form which they 

 assumed is a matter of little morphological importance. In some 

 they may have developed 



Fig. 11. — Specimen of Triarthrus 

 Beckii, showing the antennae. 



{After Beecher.) 



pincers (cf. Limulus and Ptery- 

 gotus 2 ), but in the majority 

 of cases they more probably 

 remained purely sensory. 



As to the appendages of the 

 following head-segments, we 

 should probably find every 

 grade of specialization, from the 

 lowest trilobites upward. The 

 simplest would be that stage 

 in which the head- appendages 

 did not differ either one from 

 the other or from those of the 

 trunk : all alike being, in 

 all probability, membranous 

 lobes deducible from the para- 

 podia of their annelidan ances- 

 tors. The ventral portions of 

 these were, in all probability, 

 masticatory ridges, and pre- 

 eminently specialized as such 

 in the region of the mouth. 

 Dr. "Woodward's discovery of 

 one of these head-appendages 

 in Asaphus platycephalus 3 

 shows the basal masticatory 

 ridge, while the dorsal portion 

 is developed into a jointed 

 cirrus-like process (cf. Ptery- 

 goids). In some trilobites all 



the four pairs of posterior cephalic appendages may have pre- 

 sented this character, the masticatory plates being about equally 

 developed (as in Limulus), whereas the dorsal portions were either 

 sensory organs or walking-limbs. The great interest which attaches 



1 [While this paper was passing through the press, a paper appeared by 

 Walcott, 'Note on some Appendages of the Trilobites,' Geol. Mag. June 1894, 

 p. 246, which contains a figure of a Triarthrus, showing the attachment of these 

 antennae in exactly the position which the first antennas occupy in Apus. As 

 to the great importance of this, see my note in 'Nature,' vol. xlviii. (1893) 

 p. 582.— H. M. B., June, 1894.] 



2 And, according to Laurie, Slirnonia, ' The Anatomy and Relations of the 

 Eurjpteridae,' Trans. Roy. Soc. Edin. vol. xxxvii. pt. ii. (1893) p, 509. 



3 Quart. Journ. Geol. Soc. vol. xxvi. (1870) p. 486. 



