On the structure and classification of the Tremataswdae. 23 



IV. Summary and Conclusions. 



Our observations on tlie structure of Tremataspis have brought out the following 

 principal facts : 



A. The Lateral Line Organs of Tremataspis consist of a series of shallow, groove- 

 like dots and dashes, arranged in linear series on the dorsal surface of the shield. We 

 distinguish a circum-orbital, marginal, anterior transverse, and a posterior dorsal line. The 

 first two lines appear to correspond with 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 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, when 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. 



G. 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 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. 



J). 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. 



