The Structure of Dinichthys 



139 



four'cornered, smooth on the outside and of nearly the same breadth on the whole length. 

 Its form on the inside will be described later. 



The handle of the sub-orbital presents a more complicated picture. It is composed 

 of two sharply differentiated parts: one, the real handle, is formed of the immediate 

 prolongation of the blade and is composed of a long, narrow piece with nearly parallel 

 upper and lower margins (Text-figure 21 A). In about the middle of the upper margin is 

 found a well marked angle (Text-tigure 21 a). From this pomt on, the handle gets nar- 

 rower and its upper and front margin becomes strongly thinner (Text-figure 21 [)rn and 

 pn). This part of SO comes in contact with PrO and PAJ. The other part of the handle 



Text -figure 22. 

 The right sub-orbital plate of Dinichthys intermedius Nwb. viewed from the inside. 

 A, real handle; B, tongue-shaped part of handle; gr, groove on under side of tongue-shaped part; if>, impression between curving 

 points of ridges R, and R.,; If, lower hind corner of SO. of, upper hind corner of SO: oh, upper front corner of SO; pn, contact 

 point with post-nasal plate; Pr, high crest of R;, ridge; pro, contact point with pre-orbital plate; pso, part overlapped by post- 

 3ub-orbital plate; R,, R^., R,, R^, ridges on the side of SO; sp, split between real handle A and tongue-shaped part B; y, curving 



point of ridge R , . 



is tongue-shaped and is placed lower down (Text-figure 21 B). It begins nearly at the 

 base of the handle and runs forward in the form of an arch. The distance between this 

 part and the real handle is greatest in the back and diminishes forward (Text-figure 

 23). In front these parts are divided from one another by a deep split (Text-figures 21 and 

 22 sp). The tongue-shaped part serves to attach the upper "tooth" to the suborbital. 

 A sensory canal (Text-figure 21 IX) runs from the upper front corner of the blade 

 (Text-figure 21 oh) downward nearly parallel to its margin. Not far from the lower 

 margin of SO, this canal curves sharply forward at an angle of nearly 90° from its original 



