18 William Patten. 



tube in the frontal opening of Tremataspis and the long slender ingrowth between the me- 

 dian eyes of Bothriolepis (Whiteaves 86, PI. VI, fig. 2) suggest the 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 the minute structure of the 

 shield elsewhere, and shall confine myself here to a description of certain structures seen in 

 cross sections of the whole head on the inner surface of the shell. 



Herr Akademiker F. Schmidt generously allowed me to use for sectioning two frag- 

 ments of the dorsal shield of Tremataspis Schmidti belonging to the Petersburg Akademy. 

 The most successful results were obtained from a specimen with a well preserved shell, but 

 one in which the whole frontal region and a large part of the right side were absent. Both 

 of the so called ((endolymphatic ducts» were, however, present. The fragment was cut into nine 

 sections, from which I was able to make out the following points : 



The margin of the head in cross sections is slightly thickened and triangular in out- 

 line (fig. 3, PI, I), the marginal serrations being on the ventral angle, except in the 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 region 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 strengtiien the margin, and, in general appearance are like 

 those we have described in the margin of the head shield of Limulus. The marginal trabe- 

 culae are best developed in the cornual region, 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 iuner surface of the dorsal shield. Each plate begins 

 as a well defined rounded ridge just in front of the occipital crest and close to the median 

 line. They diverge toward the anterior end and become deeper and stronger, ending in a 

 tuft of irregular, sinew-like processes. Each plate is deepest about midway between the an- 

 terior and posterior ends, and sections at this point (fig. 6, PI. 1.) 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 



