On THE STRUCTURE AND CLASSIFICATION OF THE TrEMATASPIDAE. 
23 
IV. Summary and Conclusions. 
Our observations on the structure of Tremataspis hâve brought out the following 
principal facts: 
A. The Lateral Line Organs of Tremataspis consist of a sériés of shallow, groove- 
like dots and dashes, arranged in linear sériés on the dorsal surface of the shield. We 
distinguisli a circum-orbital, marginal, anterior transverse, and a posterior dorsal line. The 
first two lines appear to correspond witli the circum-orbital and the trunk line of Pterichthys. 
The circum-orbital line is represented in Tolypaspis by a V-shaped ridge. 
B. The Sensory Openings of the Dorsal Shield: 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 the floor is an extension of the inner 
layers of the shell. 
In transverse sections through the frontal dépréssion, the median slit is seen to be a 
true perforation of the shell, its infolded margins forming a short flattened tube. 
The median orbits, wlien sectioned or excavated, are seen to be 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. 
C. The Lateral 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 indicating the presence of several polygonal plates covering the 
openings. The lateral eye orbits agréé 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. 
I). The Posterior Marginal Openings agréé in position with the so called dorsal orgaus 
of Limulus, a pair of segmentai sense organs, serially homologous with the lateral eyes, and 
lying in larval Limuli opposite the fourth pair of thoracic appendages. 
