REPORT OF THE DIRECTOR I919 



73 



Somewhat different characters suggesting atavistic tendencies have 

 been observed by Doctor Clarke in Devonian trilobites (1913, p. 

 135)- These consist in lateral spines on the cheeks in trilobites of 

 the Phacops group : namely, Proboloides cuspidatus 

 Clarke ; and spinules on the basal margin of a cephalon of D a 1- 

 m a n i t e s sp : this, hov^ever, being observed in but one case and not 

 confirmed by other specimens. Doctor Clarke says regarding 

 P r o b o 1 o i des cuspidatus: 



Fig. 14 



Fig. 13 



Fig. 13: Proboloides cuspidatus Clarke ; fig. 14 Dalmanites 

 (Mesembria) sp.? (after Clarke). 



" The elemental or reversional character of this entire group is 

 expressed here by the singular and unparalleled reappearance of 

 spines on the lateral margins of the cephalon at the outlet of the 

 facial sutures and as these lie above the normal position of the spines 

 at the genal angles, each cheek thus bears a pair of spines. This 

 remarkable structure, the significance of which is more fully esti- 

 mated in another place occurs in young and mature individuals alike 

 so that it is in no wise an indication of uncompleted ontogeny but 

 distinctly atavistic and due apparently to the reversional tendency 

 which has accompanied or resulted from geographic isolation." 



On page 22, op. cit., it is stated of Proboloides that it presents 

 an unexampled retention, or reappearance, of primitive structures in 

 its sutural spinules above the cheek angles, a structure not repre- 

 sented elsewhere above the Cambrian. Walcott (1916, p. 237) 

 has more recently in his monograph of the Mesonacidae pointed out 

 the reversion of this character of the Cambrian ancestors, as seen in 

 a number of later trilobites. He states : 



" The genal, intergenal and antero-lateral spines of the cephalon 

 undoubtedly represent the pleural ends of segments that have been 



