18 
William Patten. 
tube iu the frontal opening of Tremataspis and the long slender ingrowth betvveen the mé¬ 
dian eyes of Bothriolepis (Wbiteaves 86, PI. VI, fig. 2) suggest tbe possibility that struc¬ 
tures of a similar nature may be present in the Ostracoderms. 
The Inner Surface of the Shield : I propose to treat of tbe minute structure of the 
sliield elsewhere, and sball confine myself liere to a description of certain structures seen in 
cross sections of the whole liead on the inner surface of tbe shell. 
Herr Akademiker F. Schmidt generously allowed me to use for sectioning two frag¬ 
ments of the dorsal shield of Tremataspis Schmidti belonging to tbe Petersburg Akademy. 
The most successful results vvere obtained from a specimen with a well preserved shell, but 
one in which the whole frontal région and a large part of the right side were absent. Both 
of the so called «endolymphatic ducts» were, however, present. The fragment was eut into nine 
sections, from which I was able to make out the following points : 
The margin of the liead in cross sections is slightly thickened and triangulär in out¬ 
line (fig. 3, PI. I), the marginal serrations being on the ventral angle, except in tbe cornua 
where they lie on the dorsal wall of the head. 
The dorsal and ventral shields are united near the margin by a spongy network of os- 
seous trabeculae, similar to those seen in the orbital région and on the floor of the marginal, 
and post-orbital, openings. These trabeculae form a narrow strip that extends completely 
round the head, except possibly along the anterior margin, a part not included in my 
sections. 
The trabeculae serve to strengthen the margin, aud, in general appearance are like 
those we hâve described in the margin of the head shield of Limulus. The marginal trabe¬ 
culae are best developed in the cornual région, and there are indications that they surround 
a large marginal canal similar to that in the abdominal shield of Limulus . 
The shell is strengthened along the occipital crest by a thickening of the cancellous 
tissue of the middle layer. A median canal, probably originally filled with a large blood 
vessel, extends forwards along the occipital crest, sending off branches right and left to the 
tissue within the shell. 
The Entapophyses : On either side of the median line is a narrow plate of bone extend- 
ing downward and outward from the inner surface of the dorsal shield. Eacli plate begins 
as a well defined rounded ridge just in front of the occipital crest aud close to the median 
line. They diverge toward the anterior end and become deeper and stronger, ending in a 
tuft of irregulär, sinew-like processes. Each plate is deepest about midway between the an¬ 
terior and posterior ends, and sections at this point (fig. 6, PL I.) show that its free inner 
edge is slightly bifurcate. 
The processes appear to be ingrowths from the inner layer of the shell only, for there 
are no indications of their presence on the outer surface. They are symmetrical in outline 
and position, and there can be no question that they are normal structures. They must not 
