Vol. 50.] SYSTEMATIC POSITION OF THE TRILOBITES. 415 



corporation of somites, which is quite obscured in the development 

 of the Crustacea, is still perfectly clear in the development of the 

 trilobites, e. g. in that of Olenellus described and figured by Walcott. 1 

 This trilobite, with five head-segments in the adult, arose almost 

 certainly from a form with four head-segments ; the youngest stage 

 observed has only four segments, with their own characteristic 

 pleurae, the posterior pairs being bent backward, as terminal pleurae 

 usually are (fig. 3). When the fifth head-segment appears, it does 

 so as a trunk-segment, i. e. with typical trunk-pleurae ; that is, 

 with pleurae which run out laterally in the transverse plane (see 

 figs. 4 & 5, p. 416). These pleurae of the fifth head-segment only 

 gradually become incorporated into the head-shield, and in some 

 species their points seem to persist on each side in the middle of the 

 posterior margin of the cephalic shield. 



These figures of the developing Olenellus are further of special 

 interest because they show without doubt that the segmentation of 

 the head was still very distinct, 



i. e. the fusion of the segments Fig. 3. — Youngest stage o/Olenel- 

 was only of recent occurrence. lus asaphoides (j mm.) seen 



We find the head-segments by Walcott. 



diminishing in size from front 

 to back (fig. 4 a, p. 416), which 

 is typical of the development of 

 segmented animals when the 

 segments do not belong to a 

 highly specialized region. This 

 early developmental stage no 

 longer appears in the metamor- 

 phoses of the Crustacea. 



It appears to me, then, that t This . sh °T 8 the , f ° ur ^-segments 

 , ri . ., . ., I . ,,. with the anal segment ; the cepha- 



we have, in the trilobites Micro- lic shield appar ently consists of the 



discus and Olenellus, two con- pleurae of the lst-4th segments.] 



secutive stages in the develop- 

 ment of the crustacean head. But, at the same time, although 

 Microdiscus, with its head of four segments, is, in this respect, an 

 older type than Olenellus, with its head of five segments, in other 

 respects (for example, in its pygidium) it is more specialized. 



III. The two chief characteristics of the head of these primitive 

 Crustacea are (1) the bending round ventrally of the first segment, 

 so that the labrum and mouth face posteriorly ; and (2) the cephalic 

 shield. 



1. In my endeavour to trace the possible origin of the Crustacea 

 from their annelidan ancestor, I laid special stress upon this bending 

 round of the mouth for the purpose of using the parapodia as mouth- 

 organs. I had shown, at first without reference to the trilobites, 



1 'Fauna of the Olenel/us-zone' (see especially pi. 4, xxxvi.) in U.S. Geol. 

 Surv. Tenth Report (1890). See also S. W. Ford, ' Embryonic Forms of Trilo- 

 bites,' Ainer. Juurn. Sci. ser. 3, vol. xiii. (1877) p. 265, and vol. xxii. (18S1) 

 p. 250 ; some of Walcott's figures are taken from these papers. 



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