
No. 425. THE TREMATASPIDA. 8 
393 
circumorbital, marginal, anterior transverse, and a posterior 
dorsal line. The first two lines appear to correspond with the 
circumorbital, and the trunk line of Pterichthys. The circum- 
orbital line is represented in Tolypaspis by a V-shaped ridge. 
The Sensory Openings of the Dorsal Shield. — There are four 
centrally placed openings and two pairs of marginal ones on 
the dorsal shield of Tremataspis. Interpret these openings 
provisionally as follows : the posterior central opening is that 
of an olfactory organ; the three anterior central ones belong 
to the median eye; the anterior lateral openings contain the 
lateral eyes, and the posterior lateral pair, a segmental sense 
organ comparable with the dorsal organ of embryo Limuli. 
The olfactory opening and both pairs of lateral openings in 
all well-preserved specimens possess sharply scalloped margins 
- and a reticulated bony floor. Transverse sections show that the 
floor is an extension of the inner layers of the shell. 
In transverse sections through the frontal depression the 
median slit is seen to be a true perforation of the shell, its 
infolded margins forming a short flattened tube. 
The median orbits are separate, nearly spherical chambers, 
enclosed in a network of bony tissue formed by ingrowths of 
the inner layers of the shell The narrow median canal that 
appears to connect the two orbits is closed by a deep-lying 
bony floor, also formed from the inner layers of the shell. 
The /ateral eyes were small and subordinate in function to 
the median ones. They probably occupied the anterior pair 
of marginal openings, the rounded incisions on the edge -of the 
openings possibly indicating the presence of compound eyes, 
consisting of a few large ommatidia. The lateral eye orbits 
agree with those of Limulus in being shut off from the interior 
of the head by a bony network arising from the inner layer of 
the shell. 
The posterior marginal openings agree in position with the 
so-called dorsal organs of Limulus, a pair of segmental sense 
organs serially homologous with the lateral eyes and lying in 
larval Limuli opposite the fourth pair of thoracic appendages. 
The Median Eye.— The three anterior median openings I 
have compared with the triocular median eye of Limulus, Apus, 
