F. R. Cowper Reed — On the CheiruridcB. 211 



and it can easily be derived from that of a typical Cheirurus by 

 a broadening and flattening of its several parts. 



Turning now to another branch of the family, we find in 

 Splicer ocorypTie the forerunner and ally of the peculiar genus 

 Deiphon. The relations of SphcsrocorypJie to Cyrtometopus have 

 previously^ been discussed. Some of its leading characteristics 

 we find repeated but more accentuated in Deiphon. Thus, in 

 SpJicerocoryphe the enormously inflated anterior portion of the 

 glabella and the faintness of the first and second side-furrows 

 foreshadow the condition found existing in Deiphon, in which 

 the two pairs of furrows have disappeared and the glabella has 

 a regular globular form. In several species of Sphcerocoryphe (as, 

 for instance, Sph. Hiibneri and Sph. unicus) the basal lobes of the 

 glabella are distinct, but in Sph. cranium they are very faint, and 

 thus prepare us for their complete absence in Deiphon. Again, in 

 Sphcerocoryphe the free cheeks are merely small triangular plates, 

 bearing the eyes, wedged in on the anterior border of the fixed 

 cheeks ; in Deiphon the reduction in size of the free cheeks has 

 proceeded so far that they are only represented by the eyes and 

 a small part of the doublure of the head-shield. The fixed cheeks 

 in Sphcerocoryphe bear one or more tooth-like processes on their 

 front edge ; in Deipjhon the fixed cheeks are so narrowed and 

 modified as to form long spines, but they still bear the tooth-like 

 process on their front edge, as in Sph. granulatus. In the thorax we 

 find on the pleura in Sphcerocoryphe a longitudinal furrow, and in 

 the Bohemian individuals of Deiphon Forbesi there is also a similar, 

 though fainter, furrow present.'^ The reduction in the number of the 

 body-segments to nine is certainly in this genus, as in Sphcerexochus, 

 a sign of high specialization when we consider it in conjunction with 

 its other characters, but the loose build of the body and absence of 

 fulcrum seem to be reversions to the more archaic types, and to 

 represent the gerontic and degenerate stage in the phylogeny of the 

 group. The migration of the eyes forwards to the anterior edge of 

 the head-shield is also distinctly a retrogression to the larval con- 

 dition, for it has been proved ^ that the eyes first appear on the 

 anterior margin of the dorsal shield in the protaspis, and move 

 backwards in subsequent stages of growth. 



The pygidium in Deiphon has only one pair of pleurae — the first 

 pair ; but this pair is enormously developed. The pleurse of the 

 other segments are aborted ; but four segments are traceable on the 

 axis. This great development of the first pair of pleurae is likewise 

 foreshadowed by Sphcerocoryphe, for in Sph. unicus the first pair is 

 much enlarged, while the posterior ones are reduced. There seems 



1 Geol. Mag., Dec. lY, Vol. Ill (1896), p. 118. 



- Salter says (Mon. Brit. Trilob., p. 88) that the pleurae are imgrooved, but 

 in his figures of the species (ibid., pi. vii, figs. 1-12) furrows are shown. Barrande 

 both describes and figures the furrows (Sj'st. Sil. Boh., vol. i, Suppl., p. 115, 

 pi. ii, figs. 19, 20). Perhaps, as Barrande suggests, for this and other reasons the 

 Bohemian and English species are not identical. 



2 Beecher, Anier. GeoL, vol. xvi (1895), p. 177, and references. 



