286 H. S. Williams — Ventral Plates of the Holonema. 



the posterior part of the carapace, speaking of the plates a. v. I. 

 (fig. 2, beyond) as " posterior lateral " dorsals. My first im- 

 pression was that the plates were ventral; and in a letter from 

 Dr. Newberry the same interpretation of the plates was made. 

 " I imagine your plates are from the plastron," . . . " there 

 should be two other plates in the plastron overlapping the 

 posterior end " of the plate marked m. v., which he spoke of 

 as the " central plate of the plastron ;" and he further said, 

 " your plate A (a. m. v. of fig. 2) is altogether new." With 

 this conflict of opinion and knowing then of no analogous 

 arrangement of plates by which to settle the question, I left 

 the matter for further light. The restoration of the ventral 

 plates of the carapace of Phlyctcenaspis acadica Whiteaves, 

 by Traquair,* furnishes the needed clue. Whiteaves had 

 described the dorsal part of the carapace as Coccosteus aeadicus, 

 and had figured two plates which he referred to the ventral 

 part of the same. His specimens were from the lower Devon- 

 ian of Cambeiltown in Canada, f Dr. Traquair on examining 

 specimens from the same locality discovered a large portion 

 of the ventral shield, with nearly all its plates in situ, and 



Restoration of ventral plates of 

 Phlyctenaspis Acadica Whiteaves, after 

 Traquair. Reduction about -£-. 



Restoration of ventral plates of 

 Holonema rugosa Claypole. Reduced 

 to £ nat. size. 



from it, with the aid of detatched specimens, the restoration 

 was made, of which a copy is given in figure 1. Examination 

 of the restoration and its description shows the probable rela- 



*Geol. Mag., 1893, p. 148. 



f Trans. Roy. Soc. Canada, vol. vi, sect, iv, p. 3. pi. ix, with a woodcut. 



