THE AM ERIC AX NATURALIST. [Vol. XXXVII 



sidering. His specimen was only partly exposed and he failed 

 to see many important details. He looked on it as the anterior 

 median oral plate. I have examined Rohon's specimen and 

 there is no doubt that it is identical with the one in the Dart- 

 mouth collections. Rohon states, p. 188, 



« Die fur die erste Reihe von mir angenommenen zwei vorderen Seiten- 

 platten sind am Original nicht aUein zerbrochen, sondern auch ganzlich 



die in den Figuren 12 und 13 a, b abgebildeten und isoliert gefundenen 

 tur^ach mit den ubrigen Platten voUkommen uberein. Dass jedoch die 



und dunner erscheinen. Im Ubrigen betrachte ich, wie gesagt, das 

 Thatsiichliche in Bezug auf die beiden vorderen Seitenplatten als etwas 



We must take exception to Rohon's statement that the so-called 

 median oral plate agrees completely in form and microscopic 

 structure with that of the remaining oral ones. In the Dart- 

 mouth specimens, the surface is very smooth and shining 

 whereas in the cast of that specimen in St. Petersburg which 

 shows nearly all the plates "in situ" the surface of the oral 

 plates is broken into very delicate ripple marks similar to those 

 usually seen on the dorsal and ventral surfaces of the buckler. 

 Moreover, the isolated so-called median plate has rounded out- 

 lines and a strongly convex surface so that it would appear quite 

 out of place among the true oral plates which have nearly 

 straight line contours and flat surfaces. 



The smooth surface texture of this plate may be a specific 

 difference, as I have seen some specimens of Tremataspis with 

 very thick, strongly arched bucklers and smooth surfaces. They 

 appeared to belong to a different species from any of those 

 already described. However, a small recently discovered iso- 

 lated plate belonging to the oral region shows a smooth polished 

 surface similar to that of our shield-shape plate, indicating that 

 the surface texture of the oral plates may have been slightly 

 different from that on the rest of the body. 



