THE MODERN LIMULUS DESCRIBED. 185 



just at the highest central point, the ocelli are placed (PI. XXXVI, fig. 1, a 1), whilst the 

 larger compound eyes (PI. XXXV, fig. 2, and PI. XXXVI, fig. 1, 1, 1,) are situated nearer 

 the sides of the shield, and occupy a point midway upon the two lateral ridges which 

 encircle the central elevated tract or glabella. 1 The front of this great buckler slopes rapidly 

 down to the curved anterior border, the edge of which is strengthened by a bead-like 

 rim. The two genal or lateral portions of the shield are widely produced backward, 

 considerably over- reach the margins of the succeeding thoracic segments, and terminate 

 upon each side in a broad recurved spine 



The posterior border of the head -shield is deeply emarginated, and is articulated to 

 the anterior border of the thoracico-abdorainal segments by a broad median joint capable 

 of considerable flexure, the post-cephalic segments in the adult modern Limulus being 

 soldered together into one nearly homogeneous buckler, and only indicating their original 

 free and movable condition in the young state by the alternation of spines and rounded 

 indentations on each lateral margin, in the hollows of which a series of six spines are 

 articulated on either side in the adult (see PI. IX, fig. 1, PI. XXXV, fig. 1 b, PL XXXVI, 

 fig. 1 b). In the deep notch formed by the prolongation of the two latero-posterior angles 

 of the hinder border is articulated the powerful triangular " telson " or tail-spine, which 

 often more than equals the length of the rest of the body. The post-cephalic shield is 

 strongly trilobed, and marked by a double row of small oblong indentations corresponding 

 with the entopophysial processes, which are developed from the inner and under surface 

 to support the branchigerous feet. 



The length of the head-shield and of the post-cephalic shield is nearly equal, but the 

 former is much broader than the latter. 



Beneath the head-shield, which is a double buckler — between the inner and the outer 



1 In the Introduction to his 'Monograph on the British Trilobites,' Pal. Soc, Part I, 1864, p. 10, 

 Mr. Salter observes, " The curious so-called facial suture, a line of division which is only faintly indicated 

 in Limulu?, and which has, perhaps, no other representative in the whole Crustacean class, sufficiently 

 distinguishes the Trilobite." 



The relative position of the eyes in Limulus upon the ridge which circumscribes the glabella or 

 central portion of the shield favours this homology, and although no actual line of suture can be traced 

 dividing the general portion of the head-shield from the glabella in Limulus, nor any indication of a 

 disposition to divide along this line, as is commonly the case in Trilobites, we may conclude that they do 

 actually correspond. 



Nor is this structure of the head-shield so exceptional as Mr. Salter had supposed. For, whilst 

 Professor Dana and Mr. Spence Bate have shown that the antennal segment constitutes the anterior and 

 upper portion, and the mandibular segment forms the posterior and lower portion of the carapace in the 

 Macrouran and Brachyuran Decapod, the latter ('Annals and Mag. Nat. Hist.,' 1855, 2nd series, vol. xvi, 

 p. 36) has affirmed that the suture which traverses the lower surface of the carapace of the latter forms 

 the line of demarcation between the third and fourth somites, and is homologous with the cervical suture 

 in the Macroura, with that which traverses the dorsal surface of the cephalou in several genera of 

 Trilobites, and, we would add, of Limulus also. In this view of the structure of the head in the Crustacea 

 we cordially agree. [See also Spence Bate, 'Brit. Assoc. Reports,' 1875, Bristol, p. 46.] 



