FOSSIL FISHES. 621 



In the former of the two volumes above quoted the five plates which 

 were originally supposed to constitute the "plastron" are figured in out- 

 line (p. 10) and fully described. The cut is here reproduced. 



Ven'.ral Armor of Ditr'chthys Terrelli. N. Dr. Newberry's first restoration. 



The upper pair are designated as "anterior ventrals," the lower pair 

 "posterior ventrals,'' while the narrow one between the anteriors is the 

 "ventral median" plate. The general correspondence of this plastron 

 with that of Coccosteus as restored by Pander is noted, as well as the 

 divergencies, which latter are greatest in the posterior pair. 



Prior to the publication of the Monograph referred to above (1889), 

 the true position of the so-called "posterior ventrals" was found to be 

 upon the sides of the head, where they constituted the "sub-orbitals," 

 and to this position they are assigned in the latter work. The place 

 thus left vacant in the plastron is not positively supplied by anything de 

 scribed in the Monograph. The subject is alluded to however as follows: 

 ''No figure is given of the plates which are supposed to have formed the 

 posterior half of the plastron, because no perfect ones have been found, 

 but I have numerous fragments of relatively large plates which must 

 have been oblong in form and had the moderate and uniform thickness 

 and plainness of surface which characterize the plates that defended the 



