14 William Fatten. 



two large notched plates in the first row of Tremataspis (see the separate plates, Rolion '93), 

 and the so called maxillae of Plerichthjs. 



Moreover, according to Whiteaves, there is in Bothriolepis a triangular depression be- 

 tween the maxillae that suggests the oral depression in Tremataspis, and back of this depres- 

 sion there is a rounded median plate (18) that is not unlike the large triangular projection 

 from the anterior margin of the ventral shield of Trematapsis. 



The resemblances Ave have pointed out are obscure, but it is not improbable that fur- 

 ther discoveries may make them more manifest. 



The large notched plates of PtericMhys and Bothriolepis referred to above, are usually 

 described as the lower jaws or maxillae. «But it is clear», as Traquair has already pointed 

 out (p. 488) othat their mode of working must have been rather different from that of the 

 mandibles of ordinary Vertebrates». 



What that difference is, Traquair does not state, which is somewhat surprising in view 

 of the importance of any clue bearing on the systematic position of these animals. Obvious- 

 ly the difference is this : this so called lower jaw consists of two separate pieces that could 

 not work freely forward and backward in the sagittal plane like the jaws of «ordinary Ver- 

 tebrates)), but they could work against each other in a transverse plane, like the mandibles 

 of an Arthropod. 



It seems to me therefore, that the mouth of Plerichthjs and Bothriolepis was not a large 

 transverse opening in front of the «mental plates», but rather a small circular one, between 

 or behind them, like the mouth of Tremataspis, 



The Median and the Lateral Eyes and the Olfactory Organs : The dorsal surface of Tre- 

 mataspis presents for consideration three paired and two unpaired openings, that were with- 

 out doubt occupied by some kind of sense organs. I regard the three anterior median open- 

 ings (the ((frontal organ» and lateral eye orbits of Authors) as parts of the parietal eye 

 complex, and the whole eye as comparable with the tri-ocular median eye of Limulus, Tri- 

 lohites, Merostomata, and mangy other Arthropods. The anterior lateral openings contained 

 the lateral eyes ; and the posterior lateral pair, a segmental sense organ comparable with the 

 dorsal organ of embryo Limuli. The post-orbital opening was probably the site of the ol- 

 factory opening. 



The Post-Orbital Opening: Mitteloffnung, Schmidt; Parietal Organ of Rohon; Ol- 

 factory Organ, Patten. I am able to confirm Rohon's description of this opening and to add 

 a few minor details. It is a bi-convex opening through the outer layers of the shell, situated 

 just behind the orbits. Its margins are smooth, polished and scalloped as though the open- 

 ing were filled originally with a number of circular discs or plates. In the most perfect 

 specimens, I have counted eight of these incisions, one at either end and three on each side. 

 In some apparently very old individuals, there appear to be four or five incisions on a side. 

 In such cases the margin is rougher and somewhat wrinkled. The opening is always closed 



