()1 



line, then lengthens above and contracts into a comparatively 

 small trianj^ular summit. Ridges radiate from the center of 

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

 ridges run fi-oni the superior angles to the point of columnar 

 attachment. There is a single or double row of ])ores accom- 

 panying these ridges, but none elsewhere on the jilates. 



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

 hexagonal cujj rather more than one fourth tlie length of Ihe 

 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 centi'al ))art, some of them pyra- 

 midal, .so that a transverse section at this ])lace is hexagonal. 

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

 size and ditferent 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 subquadrangula}- plates become hexagonal and the 

 subpentagonal plates become heptagonal, octagonal and non- 

 agonal. 



The summit is subtriangulai-, flat centrally, and the jilates 

 turn over the margin in some jjlaces and in others stand u]) 

 over the arm openings so as to liide them from a summit \ ii-w. 

 The vault is covered with a large centi-al platt^ sun'oundcd by 

 eight plates and a few small ))lates 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 

 tliii'd range of body ])lat('S at the middle of one side of llie 

 triangle and another at the midill(> of another side, and another 

 against a subpentagonal plate near the middh' of the othei- 

 side and adjoining the mouth. The mouth is a i-ound 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 i)lates, two of the plativs belonging to 

 the circle of eight plates on the summit aljut ujion it. and om' 

 plate inserted between the thii-d i-ange and the circle of eight 

 plates abuts against it. The illustration of the azygous sid(> 

 view shows the mouth iind the plates suri-ounding it. Tlie 

 plate on the left is tin' one intei'cahiled Ijelwecn the raugc and 

 circle of ])lates. 



