On тле structure and classification of tue Tremataspidae. 
15 
below by a raeshwork of bony tissue directly continuons with the inner layers of the sliell. 
But wliile the inner layers of the Shell usually consist of regulär laminae of bony tissue per- 
forated by comparatively small vertical canals, the floor of the post-orbital opening hasaquite 
different color and texture. It is lighter yellow and less compact, cousisting of a spongy 
lattice work of bony trabeculae surrounding numerous large, irregulär, rounded openings, 
whose diameter may t be greater than that of the enclosing trabeculae. These openings no 
doubt serve for the passage of blood vessels and nerves to some organ lying above 
the floor. 
The trabecular bony tissue extends forwards beneath the inner sliell layer and appears 
to be continuous with the trabeculae that form the walls to eacli median orbit and the floor 
to the narrow space between them. 
The post-orbital opening in Tremataspis is without doubt homologous with the post¬ 
orbital valley of the Cephalaspidae. One of the newer specimens of C. Agassizi in the British 
Museum lias behind the orbits a long oblong opening with very clear eut margins. The opening 
is closed below by a large mass of bony trabeculae, resembling the sheet of bony tissue be¬ 
neath the post-orbital opening of Tremataspis and that beneath the median eyes of Limulus. 
The Anterior Marginal Openings, or Lateral Eyes : On the margins of the dorsal shield 
are two pairs of openings similar in character to the post-orbital one. The smaller anterior 
pair lies about opposite the frontal opening. In what appeared to be a young specimen, the 
margin of each opening had but two semi-circular incisions on a side ; in another case, there 
were two on either side and an unpaired one behind. In an old specimen in the Dart- 
mouth collection, there are four incisions on each side of the left opening. On the right 
side of the same specimen, there are apparently the same number of scallopings, but tliey 
are obscured by a* fine crenulation of the polished margin that appears to be characteristic 
of old specimens. 
The Lateral Eyes of Tremataspis agréé approximately in position and outline with 
those of an Arthropod, and they especially resemble those of Limulus in the possession of 
a bony floor formed from the inner layers of the shell. But they are unlike the usual Ar¬ 
thropod eye in the flatness of the orbits and in the absence of any élévation along the 
margin. This is perhaps due to the degenerate, or rudimentary, condition of the organ and, 
judging from the number of incisions on the margin, to the small number of ommatidia that 
must hâve been present. 
At least one pair of the marginal openings of Tremataspis are present in Gephalaspis. 
They were oblong openings of enormous extent, reaching from the cornua almost to the an¬ 
terior median point of the shield. They were closed externally by a large number of poly¬ 
gonal plates. A specimen of C. Murchisoni in the British Museum (P. 6260) shows the out¬ 
line of these openings, or areas, with great clearness. There is an oblong mass of reticulated 
bony tissue that separates the opening from the interior of the head. This extraordinary 
