THE MYRIAPODA OF NORTH AMERICA. 143 



Opisthemega Wood has principally been used in place of Theatops 

 because Newport based his genus upon an erroneous character — the 

 possession of an ocellus. In his description of Theatops, Newport 

 says: "Ocelli distincti" ; while in the description of its species, he says: 

 "Ocellis inconspicuis, lateralibus." In these two statements Newport 

 seems to contradict himself and to be uncertain about the possession 

 of an ocellus. Probably Newport has mistaken an accumulation of 

 pigment for an ocellus, and his observations as to the possession of an 

 ocellus are erroneous, at any rate, Say did not see a trace of an ocellus 

 in the same specimens. 



The supposition that Newport has made an erroneous observation is 

 further strengthened by the fact that the possession of an ocellus is 

 the only difference between Theatops and Opisthemega. Notwithstand- 

 ing this erroneous derivations of Theatops, it must be used in place of 

 Opisthemega. 



29. Monops. 



1847. — Monops Gervais. Apteres, iv, 294 (Cryptops nigra Newport). 



This name was first used by Billb. in 1S20, as a genus of Crustacea, 

 and Monops Gervais must give place to another. 



30. Rhysida. 



1844. — Branch iostoma Newport. Trans. Linn. >Soc, 411 (lithobiodes). 

 1863. — Rhysida Wood. Journ. Phila. Acad. Nat. Sci., 40. 



As Branchiostoma Newport is preoccupied by a genus of fishes, Dr. 

 Wood has proposed the name Rhysida as a substitute, which we are 

 compelled to use in place of Hranvhiostoma. 



31. Dacetum. 



1844. — Hetcrosioma Newport. Trans. Linn. Soc, 275 (trigonopoda. ) 



1847. — Dacetum Koch. Syst. Myr., 77 & 171 (capense). 



Because Heterostoma Newport is preoccupied (Reterostoma Hart- 

 lnami, Moll., 1843) we must use its synonym, Dacet urn Koch, instead. 

 Indiana University, June 1, 1888. 



