On the structure and classification of tub Tremataspidae. 15 



below by a raeshwork of bouy tissue directly co)itinuous with tlie inuer layers of the shell. 

 But while the inner layers of tiie shell usually consist of regular laminae of bony tissue per- 

 forated by comparatively small vertical canals, the floor of the post-orbital opening has a quite 

 different color and texture. It is lighter yellow and less compact, consisting of a spongy 

 lattice work of bony trabeculac surrounding numerous large, irregular, rounded openings, 

 whose diameter may be greater than that of the enclosing trabeculac. 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 shell layer and appears 

 to be continuous with the trabeculac that form the walls to each 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 Gephalaspidae. One of the newer specimens of C Agassizi in the British 

 Museum has behind the orbits a long oblong opening with very clear cut margins. The opening 

 is closed below by a large mass of bony trabeculac, 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 tlie 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 arc apparently the same number of scallopings, but they 

 are obscured by a fine crenulation of the polished margin that appears to be characteristic 

 of old specimens. 



The Lateral Eyes of Tremataspis agree 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 elevation 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 have 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 G. 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 



