16 William Patten. 



plate is very similar extent and general appearance to the great sheet of bone covering the 

 lateral orbits of Linmlus. 



The Posterior Marginal Openings are larger and more diamond shaped than the anterior 

 ones, due in part to the presence of an unpaired anterior and posterior incision of the mar- 

 gin, like those in the postorbital opening. In the specimen copied in the reconstruction, 

 there were three incisions on each side of the left opening. The right one was similar, but 

 larger and not quite so symmetrical in outline. Transverse sections show that both pairs of 

 marginal openings are provided with a perforated bony floor, precisely like that of the 

 postorbital opening. 



The Median Eye : Let us begin with a description of the dumb-bell shaped orbital 

 openings. When the matrix is carefully removed from their polished rounded margins, it is 

 seen that the narrow median space between the two orbits is closed by a floor formed of a 

 complicated network of bony trabeculae, PI. I. fig. 2. The trabeculae lie a little below the 

 surface, are darker in color than the rest of the shield and softer in texture. In one specimen 

 there were two coarser longitudinal strands, d, running along the median margin of each 

 orbit. In sections these two strands appeared to be hollow. 



The orbits, when excavated, form two separate hemispherical cavities. They are en- 

 closed in an irregular bony lattice work best defined on the median aiid superficial boundaries 

 of each orbit where it forms a rather broad and nearly continuous belt, c. In the deepest 

 parts of the orbit, the lattice work is very thin and has large open spaces, no doubt for the 

 passage of blood vessels and nerves. The boundaries of the deep hemispherical orbits are also 

 well shown in sections, fig, 5. Rohou did not see the floor to the orbits, but he speaks of a 

 partition, and figures the same in PI, I, fig. 12, which evidently refers to the extension of 

 the inner layers across the narrow canal uniting the two orbits. 



The edges of the orbits are beautifully smooth and rounded and show no trace of a 

 central covering or lid, like that so conspicuous in Bothriolepis. 



The essential features of the median pit, as viewed from without have been well de- 

 scribed by Schmidt and Rohon. It lies just in front of the orbits and may be described as 

 a crater-like elevation, whose polished and nearly circular rim is slightly more conspicuous 

 in front, where it may be raised in some individuals into two low tubercles fig. 2, a. Within 

 the rim is a rather deep rounded depression, the floor of which is perforated by a slit-like 

 opening whose somewhat pointed anterior end is higher than the posterior, as it extends 

 well forward onto the rim between the two marginal tubercles. 



The margin of the slit is slightly raised, and on either side of its anterior end is a 

 small depression in the floor of the crater, where the shell is quite rough and full of coarse 

 pores, which make it difficult to clean out the matrix without injury. Sections show that 



