84 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



be regarded as a part of the eye segment, or acron, and further reasons 

 for so regarding it will be given later. The other four glabellar lobes 

 must then represent four primary larval somites, the intersegmental 

 lines of which should, theoretically, have extended to the lateral 

 margins of the simple oval body before segmentation in the latter was 

 suppressed. The postlarval somites of the adult trilobite are generated 

 teloblastically (fig. 36 B, C, D) from a small region of the larva 

 behind the glabella (A, ZG), and are, therefore, clearly secondary 

 somites. The definitive segments of the postcephalic series remain 

 distinct in some of the trilobites to the end of the body (E, F), where 

 there is a small terminal lobe (E, G, Tel?) that may be the telson; 

 in others the posterior segments are united in a tail-fan, or pygidium 

 (H, Pyg), and in the Agnostia (I) only two segments retain their 

 independence between the head and the pygidium. 



The Xiphosurida in the adult stage resemble the Trilobita in so 

 many respects that we should expect to find an even closer approach 

 to the trilobite structure in their developmental stages ; and, in fact, 

 it has been shown by Iwanoflf (1933) that the primary segmentation 

 in the embryo of Liinuhis moluccamts produces four somites (fig. 

 37 A, I-IV), those of the chelicerae, the pedipalps, and the first two 

 pairs of legs, which evidently represent the four postacronal head 

 somites of a trilobite. Because of the large amount of yolk in the 

 ectoderm, embryonic metamerism appears first in the mesoderm, 

 which is early divided almost simultaneously into four sections corre- 

 sponding with the four primary somites. The preoral cephalic region 

 of L. moluccanuSj IwanofT says, is at first not distinctly differentiated 

 from the surrounding blastoderm, but later it becomes apparent as a 

 preoral head segment without appendages, and in an older embryo it 

 forms a pair of definite cephalic lobes (B, Pre). Behind the fourth 

 somite there is in the young embryo (A) only an unsegmented tail 

 piece, but at the base of this region are later generated consecutively 

 (B) the remaining segments of the adult, which are thus typically 

 teloblastic in the manner of their formation. 



It would thus appear that the primary segmentation of the ancestors 

 both of the trilobites and the xiphosurids produced only four somites. 

 These four primary somites, united with one another and with the 

 cephalic lobe, or acron, constitute the "head" in the Trilobita (fig. 

 36 H, H) ; in the Xiphosurida they form the anterior part of the 

 prosoma, for in this group three following somites and part of a 

 fourth are combined with the four primitive somites in the anterior 

 section of the body (fig, 47 E). Moreover, in the Xiphosurida a 

 union has taken place between all the opisthosomatic somites, so that 



