6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 28 



being well within the limits of individual variability in conspecific 

 populations observed in similar Cambrian trilobites. From the com- 

 bined features of these four cranidia, it seems that Poulsenia is close 

 to Antagmus, the chief difference being in the slight convergence of 

 the anterior facial sutures viewed from above. No other species seem 

 to possess the same combination of characters ; hence Poulsenia is 

 provisionally restricted to the Greenland species, 



Syspacephalus Resser, 1936 (Agraulos char ops Walcott). The type 

 species is well represented and was recently refigured by the writer 

 (Rasetti, 1951) with the descriptions of several lower Middle Cam- 

 brian species. A genus rather well characterized by the anterior posi- 

 tion of the eyes and the slight convergence of the anterior facial 

 sutures. 



Onchocephalns Resser, 1937 {Ptychoparia thia Walcott). The type 

 species is fairly well known from several cranidia (Rasetti, 1951). 

 Intermediate forms link the genus with Antagmus, Crassifimbra, and 

 the Lower Cambrian species which Resser incorrectly referred to 

 Proliostracus and Phychoparella. Hence the limits of the genus are 

 to a great extent arbitrary. 



Periomma Resser, 1937 {Periomma typicalis Resser). The re- 

 marks concerning Antagmus also apply to this case. The type species, 

 represented by distorted cranidia lacking the test, is characterized by 

 the elevation of the palpebral area and of the anterior border. The 

 genus appears to be valid. 



Aiistinvillia Resser, 1938 (Austininllia virginica Resser). A valid 

 genus, based on a well-represented species. However, a second form 

 described by Resser does not appear congeneric with the type species, 

 and is here used as the type of a new genus. 



Litocodia Resser, 1938 (Litocodia typicalis Resser). Lochman com- 

 mented on the poor preservation of the type material and referred the 

 species to Antagmus. Even though this assignment is not definitive, 

 the name Litocodia should be restricted to the type species until this 

 becomes better known. 



Periommella Resser, 1938 {Periommella yorkensis Resser). A dis- 

 tinctive genus, here described in detail. 



Crassifimbra Lochman, 1947 (Onchocephalus walcotti Resser). 

 The type species, represented by good limestone cranidia, seems dis- 

 tinct enough from Onchocephalus to warrant a separate genus. How- 

 ever, intermediate forms are known. 



Piasclla Lochman, 1947 {Ptychoparia pia Walcott). The type 

 species is characterized by a relatively smaller glabella than most of 

 the other mentioned genera. However, intermediate forms bridge the 



