On the structure and classification of the Tremataspidae. 



21 



although the two post-orhital dashes, and the three pairs in the anterior transverse line, 

 are absolutely constant; at least they were so in the eight or ten specimens perfect enough 

 to show the lines clearly. 



The Circum-orbital Lines ') consist of two semicircular lines of dots and dashes almost 

 completely surrounding the frontal eminence. The first dash is invariably a long one, lying 

 just behind, and median to, the post-orbital tubercle. Then follows a rather wide interval 

 till the line begins again with a well defined row of closely set dots and dashes extending 

 in a semicircle toward a point in front of the orbits. Here the line forms a slight loop 

 forwards and then bending backwards, maybe continued in some specimens by two small dashes 

 straight across the median line. In old specimens, all the dashes of the same side, except 

 the posterior pair, may unite to form a well defined continuous groove. In such cases, 

 however, the anterior ends of each line do not become continuous across the median plane. 



In young specimens, the circumorbital lines may consist of a beautifully regular series 

 of dots and long and short dashes. 



The Marginal Line begins with a transverse dash nearly opposite the middle of the 

 post-orbital opening. The line tlien tends outwards and backwards in a graceful curve that 

 extends the whole length of the shield. The dashes in this line are long and fairly constant 

 in number and regular in size. There is a slight change of curvature, and an increased 

 space between the dashes, where this line is crossed by the anterior transverse one. I have 

 counted five dashes in front of this interruption, and seven or eight back of it. 



The Anterior Transverse Lines. There are invariably three long dashes in each of 

 these lines. They begin a little behind, and to one side of the small circular openings in 

 the shell (anterior entapophyses) and extend outwards and slightly backwards. The first two 

 dashes lie on the median side of the marginal line, and the third near the posterior median 

 side of the posterior lateral opening. 



The Posterial Dorsal Line begins in front of the crista occipitalis either as an irre- 

 gular transverse line, or as a pair of slightly bent dashes, and is continued backwards, on 

 either side of the crista occipitalis, by from two to four separate dashes. 



I have failed to find any trace of these dot and dash markings on the ventral shield. 

 In the collection of the St. Petersburg Academy, there is a triangular plate, probably 

 belonging to the oral region, whose surface was marked by a single bent groove like those we 

 have just described. 



Superficial markings of the shell, comparable with those of Tremataspis, probably 

 occur in most of the Ostracoderms. Lankester has shown in several figures oi Pieraspis and 



1) The delimitation of the linos described is perfectly 

 clear except just hehind the orbits. I could find here no 

 certain indications as to whether the two post-orbital 

 dashes represent the median ends of the circum-orbital, 



or of the marginal lines. I have assamed, mainly for con- 

 venience of description, the former to be the case, and 

 hence regard the first two pairs of dashes back of the 

 orbits as the posterior ends of the circum-orbital linear 



