On THE STRUCTURE AND CLASSIFICATION OF THE TrEMATASPIDAE. 
scopie structure of the sliield is carefully worked out, liis results agreeing substantially with 
Schmidt’s. Не distiuguishesfour layers: I)The tliiu glassy, prismatic or enamel layer; 2) the 
bony layer, containing Haversian cauals; concentric Haversian lamellae, and bone cells. 
3) The layer of large medullary chambers with fewer bone cells in the matrix, and 4) the 
inner layer, consisting of parallel lamellae of crossed fibres and elongated lacunae. Не re¬ 
gards the frontal pit of Thyestes and Tremataspis as containing an organ comparable with 
the vesicular outgrowth of the roof of the secondary forebrain (paraphysis as described by 
Selenka); and considers it probable that asimilar frontal organ occurs in Cephalaspis, Au- 
chenaspis and Pteraspis. The anterior lateral openings are regarded as nasal pits, comparable 
with the lateral openings (the so called orbits or nasal pits of Huxley) of Pteraspis. The or¬ 
bital opening indicates the presence of a primitive, unpaired eyebalî comparable with the 
originally unpaired lateral eyes of true Vertebrates. The Parietal organ (Mittelölfnung, 
F. Schmidt) is the opening for the parietal eye and is represented in G Ayassizii by the 
post orbital valley. 
The two small openings behind the parietal organ are the openings of the Ducti endo- 
lymphatici. 
The posterior pair of lateral openings is comparée! with the pseudobranchial openings 
of certain Selachians and possibly with the marginal cells of Euheraspis. The six marginal 
incisions on the anterior margin of the ventral shield are regarded as indications of an equal 
number of gill openings. 
In 1893, Rolion adds new data on the structure of the oral région and sensory open¬ 
ings, and describes the structure and arrangement of the trunk scales. The parietal opening 
and the two pairs of lateral openings, are shown to hâve scalloped margins. The parietal 
and nasal openings hâve a spongy floor formed by a continuation of the inner layer of the 
shell. Не is now doubtful about the significance of the openings. 
A part of the posterior dorsal line of surface furrows (lateral line organs) is erro- 
neously described as the last trace of a transverse segmentation of the dorsal shield. 
Не distinguishes ten polygonal oral plates, four on each side and two median ones. 
The mouth is erroneously supposed to be a large semicircular slit between the anterior row 
of plates and the anterior margin of the dorsal shield. Three rows of trunk scales are de¬ 
scribed; a dorsal, median, and a ventral, or marginal, row; also a large number of small 
scales in the caudal région. 
In 1899, Rohon maintains that the ant-orbital opening, or «frontal-organ» and the 
postorbital opening or «Foramen-parietale» of Tremataspis mark the location respectively 
of a primitive «Paraphyse» and pariétal organ, comparable with tliose openings, or organs, 
in other fossil or recent fishes, and in the Asterolepidae , Stegocephalidae and recent Saurians. 
He also maintains that the orbits of Thyestes and Tremataspis point to the origin of the paired 
eyes of recent Vertrcbrates from a single unpaired organ. 
