OF THE AMERlCAIf PIEEASPIDIAN, PAL^ASPIS. 561 



two laterals, or the anterior might be suppressed or united with 

 the dorsal. This change would bring Diplaspis as described by 

 Mr. Matthew, and Palceaspis as figured here, into line with the older 

 forms. 



With the structure of Pteraspis as indicated it is, however, 

 difficult to see any special place for the attachment of an organ of 

 motion such as I have described in this paper, supposing its 

 function to have been rightly interpreted, and I should like to at 

 least throw out the suggestion that the openings in the ' cornua ' or 

 lateral plates which have somewhat puzzled authors on this subject 

 may have answered this purpose, and may correspond with the 

 * critical point ' on which I have laid some stress in my account of 

 the genus. If I read Lankester's description aright (I have no 

 critical specimen of Pteraspis for reference) there is a passage 

 through these to the inside of the body. Indeed this must be the 

 case in order to afford a foundation for the supposition that they 

 are connected with the branchial organs, of which, however, we 

 thus far know nothing. 



XI. Amended Definition of Palceaspis. 



In conclusion, I append amended descriptions of the genus and 

 species as now known (March, 1892). 



Pal^aspis (Claypole), 1884. 



Armour consisting of a dorsal, a ventral, and at least two 

 (perhaps four) lateral plates. 



Dorsal plate oval, nearly alike at both ends, not very convex, 

 even-edged, and with rather shallow orbital notches in the margin. 



Ventral (Scaphaspid) plate (the Pteraspis bitruncata of 1884) 

 rather smaller than the dorsal, usually rather more convex, trun- 

 cate ; lateral edges not straight, but forming an obtuse angle about 

 the middle. 



Post-lateral plates elongate-triangular in outline, resting on the 

 posterior slope of the edge of the ventral plate. 



Antero-laterals suspected to exist, but not proved. 



No rostral plate or indication of one by striation or otherwise. 



PaL^ASPIS AMERICANA. 



Dorsal shield oval, about 3 to 4 inches in length. 



Ventral bitruncate (P. bitruncata of 1884). 



Post-laterals pointed in front and terminating with ventral 

 behind. 



Antero-laterals perhaps present. 



" Pin," or some appendage resembling one, with fringed margin 

 (Crossopterygian), and probably attached in front of the post- 

 laterals. 



