422 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



DESCRIPTION OF PLATE 44 



PAGE 



Fig. I. Ncvadia zveeksi Walcott (pi. 23) 257 



2. Mcsoiiacis vcrmontana (Hall) (pi. 26) 264 



3. Elliptoccphala asaphoides Emmons (pi. 24) 269 



4. Callavia broggeri (Walcott) (pi. 27) 279 



5. Holiiiia kjcntlft (Linnarsson) (pi. 27) 288 



6. JJ'aiincria ivalcottaims (Wanner) (pi. 30) 302 



7. Pccdeiimias transitans Walcott (pi. ^3) 305 



8. Pccdeiimias transitans Walcott (pi. 2>2)) 305 



Enlargement of the posterior portion of fig. 7 showing the 

 rudimentary segments and pygidium beneath the telson-like 

 segment. 



9. OlcncUus thompsoni Hall (pi. 35) 336 



The above series of figures is reproduced in order to illustrate the variation 



in the principal genera of the Mesonacidse, also in order that the student may 

 at a glance note the changes from the most primitive form Nevadia (fig. i) 

 through one line of descent, as represented by figs. 2, 3, and 7, to Olcnclhts 

 (fig. 9). 



On another line of descent the figures serve to illustrate through figs, i, 3, 

 4, 5, and 6 the probable line of descent from Ncvadia (fig. i) to Holmia (fig. 

 5) and on to Paradoxides. 



