no SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



bases is not exactly known, but the antennal appendages are generally 

 represented as arising at the sides of the labrum. The four following 

 segmented, leglike appendages of the head clearly pertain to the four 

 postfrontal somites of the dorsal shield. 



Henriksen (1926), in his analysis of the segmentation of the trilo- 

 bite head, convincingly argues that the free cheeks bearing the com- 

 pound eyes must belong to the "eye segment" (acron), and that the 

 preocular parts of the facial sutures are secondary lines of cleavage 

 to facilitate moulting ; but the median part of the eye segment he 

 believes is represented only by the narrow anterior marginal rim of 

 the dorsal shield before the frontal lobe. Henriksen notes, however, 

 the anomalous position of the median eye far back on the glabella, 

 and it is not clear why the reasoning by which he assigns the free 

 cheeks to the eye segment does not demand that the eye segment 

 include also the area of the median eye. The antennae, Henriksen 

 contends, belong to a separate postoral somite, represented dorsally 

 by the frontal lobe of the glabella. Furthermore, since he believes 

 that the trilobite head must have the same segmentation as the head 

 of certain Crustacea, Henriksen concludes that a second antennal 

 somite has been lost by the trilobites. To the writer this theoretical 

 elaboration of the trilobite head to give conformity with crustacean 

 structure appears quite unnecessary, since the trilobites are non- 

 mandibulate arthropods having no immediate relations with the Crus- 

 tacea, and their structure clearly leads into that of the Chelicerata. 



The Xiphosurida, in the structure of the prosoma, show unmis- 

 takably their trilobite derivation, for the trilobite head is carried over 

 into the xiphosurid prosoma with few changes other than the inclusion 

 of a few extra segments, the loss of the antennae, and a differentiation 

 of the other appendages. 



A comparison of figure 47 A with figure 46 E will show at once 

 the likeness of the prosomatic carapace of Limulus to the typical head 

 shield of a trilobite. The segmentation of the xiphosurid prosoma is 

 evident from the position of the limb bases on the ventral surface 

 (fig. 47 C), where it is seen that the anterior somites lap forward at 

 the sides of the labrum from behind the central mouth, while the 

 posterior somites curve somewhat backward. The chelicerae (Chi) 

 thus come to have anatomically a preoral position at the sides of the 

 labrum, though their somite (/) is morphologically postoral, and the 

 same is true of the pedipalps (Pdp) and the first legs (iL). On 

 the inner surface of the prosomatic carapace the attachments of the 

 limb muscles (fig. 47 B), as depicted by Benham (1885), follow the 

 segmentation indicated ventrally by the limb bases. The cheliceral 



