112 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 75 



" Comparison of the thoracic segments brings out another difier- 

 ence. Thus, while these segments in Symphysiirina are broadly fur- 

 rowed lengthwise, those of Platycolpus are more deeply and narrowly 

 furrowed. In other words, the thora'cic segments in the former genus 

 are essentially like those in Megalaspis. In the latter genus, on the 

 contrary, they are marked much the same as in the Dikelocephalinae. 



" Despite certain similarities the two genera appear to be widely 

 distinct genetically, Platycolpus being derived from a Cambrian genus 

 allied to Dikelocephalus, while Symphysurina descended from an 

 Illaenurus-Vike type. 



" Finally, it remains to compare the new genus with the older 

 Illaenurus Hall, and the younger American species of Hemigyraspis 

 Raymond, to both of which it seems to be closely allied. In fact 

 these three genera represent stages in a single genetic line. The 

 cranidium of Illaenurus is scarcely distinguishable from that of 

 Symphysurina, except that in the latter a tendency to develop genal 

 spines is commonly manifested that has not been observed in the 

 Upper Cambrian species of Illaenurus. The real differences between 

 the two types are found in the thorax and pygidium. In Illaenurus, 

 namely, the thoracic axis is much broader and less distinctly separated 

 from the pleura, while the pygidium is relatively smaller and shorter, 

 evenly convex, and without a sign of an axis." 



Among the species now referred to Symphysurina are : 



5". f entella Walcott. 

 ^. eiigenia Walcott. 

 5". eurekensis (Walcott). 

 5". illaenoides (Billings). 

 S. spicaia Ulrich. 

 ^. woosteri Ulrich. 



In addition there are also at least 20 species in process of illustra- 

 tion and description. 



SYMPHYSURINA ? ENTELLA, new species 



Plate 21, figs. 19-24, 30 



Observations. — This species should perhaps be referred to Sym- 

 physurina with reservation since it departs considerably from the 

 other species referred to the genus. Compared with 5". woosteri 

 Ulrich, the facial suture of S. entella diverges considerably morp and 

 takes a much more convex course anterior to the eyes. It is intra- 

 marginal almost to the center. The eyes themselves are a bit smaller. • 

 The median tubercle is also situated farther back. No dorsal furrows 

 are apparent, their position being indicated by depression? where the 



