64 



line, then lengthens above and contracts into a comparatively 

 small triangular summit. Ridges radiate from the center of 

 each plate to the angles, except on the basal plates, where the 

 ridges run from the superior angles to the point of columnar 

 attachment. There is a single or double row of pores accom- 

 panying these ridges, but none elsewhere on the plates. 



The basal plates are of unequal size and form an angular, 

 hexagonal cup rather more than one- fourth the length of the 

 body. The six plates in the second range are of unequal size, 

 two are pentagonal, two hexagonal and two heptagonal. These 

 plates are protuberant at the central part, some of them pyra- 

 midal, so that a transverse section at this place is hexagonal. 

 There are eight elongated plates in the third range, of unequal 

 size and different in outline. The general form of six of these 

 plates is pentagonal and the other two quadrangular, but the 

 small plates belonging to the vault change the number of sides 

 so that the subquadrangular plates become hexagonal and the 

 subpentagonal plates become heptagonal, octagonal and non- 

 agonal. 



The summit is subtriangular, flat centrally, and the plates 

 turn over the margin in some places and in others stand up 

 over the arm openings so as to hide them from a summit view. 

 The vault is covered with a large central plate surrounded by 

 eight plates and a few small plates around the orifices at the 

 angles of the triangular summit. One of the circle of eight 

 plates abuts against a subquadrangular plate belonging to the 

 third range of body plates at the middle of one side of the 

 triangle and another at the middle of another side, and another 

 against a subpentagonal plate near the middle of the other 

 side and adjoining the mouth. The mouth is a round orifice 

 surrounded by five plates. It is situated near the middle of one 

 of the sides of the triangle, and truncates the angles of two of 

 the third range of body plates, two of the plates belonging to 

 the circle of eight plates on the summit abut upon it, and one 

 plate inserted between the third range and the circle of eight 

 plates abuts against it. The illustration of the azygous side 

 view shows the mouth and the plates surrounding it. The 

 plate on the left is the one intercalated between the range and 

 circle of plates. 



