14 
William Patten. 
two large notcbed plates in the first row of Tremataspis (see the separate plates, Rolion ’93), 
and the so called maxillae of Plerichthys. 
Moreover, according to Whiteaves, there is in Bothriolepis a triangulär dépréssion be- 
tvveen the maxillae that suggests the oral dépréssion in Tremataspis , and back of this déprés¬ 
sion there is a rounded median plate (18) that is not unlike the large triangulär projection 
from the anterior margin of the ventral shield of Trematapsis. 
The resemblances we bave pointed out are obscure, but it is not improbable that fur- 
tlier discoveries may make them more manifest. 
The large notcbed plates of Pterichthys 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) «that their mode of working must hâve been rather different from that of the 
inaudibles of ordinary Vertebrates». 
What that différence is, Traquair does not state, which is somewhat surprising in view 
of the importance of any due bearing on the systematic position of these animais. Obvious- 
ly the différence is this: this so called lower jaw consiste of two separate pièces 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 tlierefore, that the mouth of Plerichthys 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 présents for considération 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 vvith the tri-ocular median eye of Limulus , Tri- 
lobites, Merostomata, and maugy other Arthropods. The anterior lateral openings contained 
the lateral eyes ; and the posterior lateral pair, a segmentai 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 confirai 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 sliell, 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 dises or plates. In the most perfect 
specimens, I hâve counted eiglit 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 
