16 
William Patten. 
plate is very similar extent and general appearance to the great slieet ol bone covering the 
lateral orbits of Limulus. 
The Posterior Marginal Openings are larger and more diamond sbaped than the anterior 
ones, due in part to the presence of an unpaired anterior and posterior incision of the mar- 
gin, like tliose 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 botli 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 tlieir 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 liollow. 
The orbits, when excavated, form two separate hemispherical cavities. They are en- 
closed in an irregulär bony lattice work best defined on the median and superficial boundaries 
of each orbit where it forms a rather broad and nearly continuons beit, c. In the deepest 
parts of the orbit, the lattice work is very tliin and lias large open spaces, no doubt for the 
passage of blood vessels and nerves. The boundaries of the deep hemispherical orbits are also 
well showu in sections, fig. 5. Rolion did not see the floor to the orbits, but lie speaks of a 
partition, and figures the same in PI. I, fig. 12, whicli evidently refers to the extension of 
the inner layers across the narrow canal uniting the two orhits. 
The edges of the orbits are beautifully sniooth 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 hâve been well de- 
scribed by Schmidt and Rohon. It lies just in front of the orbits and may be described as 
a crater-like élévation, 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 а rather deep rounded dépréssion, the floor of wliicli 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 dépréssion in the floor of the crater, where the shell is quite rough and füll of coarse 
pores, which make it difficult to clean out the matrix without injury. Sections show that 
