8 THE DIFFERENCES BETWEEN 



of its area, is occupied by a dusky spot with short rays of 

 this color reaching out along the sutures between adjacent 

 plates. This dusky area is closely lined with obscure pale 

 stripes. 



The head of M. geographicus shows a mark behind each 

 eye comparable to that I have described for M. lesueuri. 

 It is never comma-shaped and is generally, perhaps al- 

 ways, completely isolated. It may be circular in outline, 

 but is more often triangular, elongated and placed longi- 

 tudinally on the head. It is very commonly enclosed by 

 a very faint pale line from which another narrow line ex- 

 tends backward upon the neck. A median line occurs 

 here also. It extends posteriorly from the snout, gradu- 

 ally widening to its posterior extremity, which is on a line 

 with or a little anterior to the anterior edges of the spots 

 behind the eyes. On each side of this line is a couple of 

 faint lines which in some cases turn inwards behind the 

 median line where corresponding lines unite. In other 

 cases the inner lines unite thus, while the two outer join 

 to form a single median stripe which continues posteriorly 

 upon the neck. The characteristic line of the side of the 

 head is a rather wide one which originates on the tympa- 

 num and thence extends ventral ly and posteriorly upon 

 the neck. Sometimes it gives off a spur towards the an- 

 gle of the mouth and still another at the ventral edge of 

 the mandible. A small pale dot is commonly visible be- 

 neath each eye. Spots corresponding to those described 

 as on the mandible near the angle of the mouth in the other 

 species can be made out here. But instead of the spot on 

 the symphysis is a median line which extends to the pos- 

 terior edge of the mandible where it expands and divides 

 into two lines which diverge backwards on the ventral side 

 of the neck. The expanded posterior ends of the ridges 

 of the dorsal plates are brownish-black in young, but not 



