I08 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



The so-called "head" of an adult trilobite (fig. 36 H, H), as we 

 have seen, represents the 5-segmented body of the larva (A), the 

 "body" segments of the adult being formed secondarily of a series 

 of teloblastic somites generated from a subterminal zone of growth 

 (ZG). The very young larva (fig. 46 A) presents a broad anterior 

 acronal region (Acr), and a postacronal region in which are already 

 differentiated the elevated median glabella (gib), which is the cephalic 

 part of the rhachis, and the broad lateral areas (fg) that become the 

 fixed cheeks of the adult (E). When the glabellar impressions appear 

 (B, C) the glabella is cut into five consecutive divisions, but it is 

 evident that the first division, or frontal lobe (C, frl), is derived 

 from the acron, and that the following four divisions represent the 

 first four postacronal somites {I -IV). With successive stages of 

 development (B, C, D), the lateral wings of the acron {Ig) extend 

 posteriorly along the sides of the somites and eventually form the 

 so-called free cheeks of the adult' (E, Ig), on which are located the 

 compound eyes (E). The cephalic segmentation of the trilobite larva, 

 therefore, may be represented as at I of figure 46, in which the 

 intersegmental lines {is-/j.s) are theoretically extended to the lateral 

 margins of the body. A median dorsal ocellus, when present, is 

 always situated on the glabella, but since it must belong to the acron, 

 it is placed on the frontal lobe in the diagram (I, dO). 



In the mature trilobite head of typical structure (fig. 46 E), the 

 preocular part of the acronal suture (I, is) has disappeared, but the 

 postocular parts become the posterior parts of the sutures known as 

 the "facial sutures" (fsp), the preocular parts of which (fsa) are 

 probably secondary lines of cleavage developed to facilitate moulting. 

 In some forms the facial sutures end on the lateral margins of the 

 head ; in others they go to the posterior margin (E), and in such cases 

 the genal spines are continuations of the free cheeks. On the ventral 

 surface of the head (F) the acronal surface is broadly inflected to 

 form the doublure (dbl), which carries the labrum (Lm), or "hypo- 

 stome," on its preoral margin. The probable dorsal segmentation of 

 the adult trilobite head, therefore, may be represented diagram- 

 matically as shown at J of figure 46. The acron (Acr) clearly forms 

 an extensive part of the mature cephalic structure, since it must in- 

 clude the frontal lobe of the glabella (frl), the free cheeks (Ig) with 

 the compound eyes (E), and the doublure (F, dbl) with the labrum 

 (Lm). Furthermore, since the dorsal ocellus often occurs far back 

 on the glabella (J, dO), we must assume that it is contained in a 

 median, tongue of the frontal lobe extended posteriorly into the 

 glabellar somites, because the simple eyes as well as the compound 

 eyes always belong to the acronal segment. 



