414 ME. H. M. BERNARD ON THE [Aug. 1S94, 



primitive segmentation of the head-region. It is even quite 

 obscured in Apus, and can only be gathered from the number of 

 cephalic appendages. 



The head-region of the trilobites is also, as a rule, so specialized 

 that it is no longer possible to make out its exact segmentation. 

 Although five seems to be the usual number of component segments, 

 four forming the glabella, and the fifth the ' occipital ring,' trilobites 

 occur in -which all traces of segmentation have disappeared from the 

 glabella, while again, on the other hand, others appear to have six 

 segments forming the head. Barrande has tabulated the apparent 

 segmentation of the heads of the Silurian trilobites of Bohemia 

 (vol. i. pp. 195-7). The numbers range from 2 ? to 6. There is 

 no reason why the trilobites should not show great variation in the 

 number of the segments composing the head ; indeed, the con elusions 

 at which we have arrived concerning their systematic position 

 would lead us to expect such variation. Fortunately, in the ancient 

 Cambrian forms, such as Microdiscus and Olenellus, the segmentation 

 of the head is so clear that it is almost impossible to misunderstand 

 it. A study of these forms seems indeed to show us the crustacean 

 head in making. 



Commencing with Microdiscus (fig. 2), we find that it has only 

 four distinct segments embraced 



by the head-shield. The fourth Tig. 2. — Head-shield of Micro- 

 segment, further, shows traces discus Meeki, showing head of 

 of quite recent incorporation four segments, the fourth only 

 into the head (see fig. 2, profile). partially incorporated in the 

 iSo that this form points back head. 

 to the time when there were 

 only three segments forming 

 the head-region. There are 

 other trilobites with apparently 

 only four segments in the head 

 (e. g. Triarihrus Beckii), which 

 on that account ought, perhaps, 

 to be classed with Microdiscus [From Wa i cott , Tenth Report U.S. 

 as a group distinct from those Geol. Surv. (1890) pi. lxxxi.] 

 with five segments. On the 



other hand, many trilobites with five head-segments show signs of 

 having arisen from those with only four head-segments, inasmuch 

 as the fifth very often bears the appearance of having been recently 

 incorporated ; it frequently retains its strong resemblance to the 

 trunk-segments, and is seldom completely merged with the glabella. 



We may, then, safely conclude from the study of adult forms 

 alone: (1) that Microdiscus was preceded by a form with three 

 head-segments ; (2) that forms with four head-segments, of which 

 examples such as Microdiscus have been preserved, preceded the 

 forms with five head-segments ; (3) that forms with six head- 

 segments (Ogygia and the related Limulus and Eurypterids) are 

 to be derived from those with five head -segments. 



The formation of the head-region by the fusion and gradual in- 



