84 
nites, and hence the assertion that, species of that genus occur in mountain 
and transition lime-stone. But the absence of the pentapetalous semistriated 
articulation of the columnar joints, and the different formation of the pelvis and 
plates resting on it, must remove them at once from this class, and place 
them in their respective genera. 
The Cotumnar Joints (fig. 8). are alternately thicker and thinner, but dif- 
fer very little in their width from each other ; they are externally smooth, flat, 
and acutely angular. At the articulating surface they are surrounded by a 
striated margin slightly arched, inwardly enclosing a smooth area perforated in 
the centre by a pentagonal cana!, whose points extend to the intervening spaces 
between the angles of the circumference. In some of these joints the sphinc- 
terlike contraction of the muscle near the alimentary canal, its adhesion to the 
points of the pentagon, and its connection with the joints next below it by longi- 
tudinal fibres, has produced a five-fold depression. 
The Petvis (fig. 3.) formed of three plates is cup-shaped, sustaining five 
long scapule (fig. 5.) each having a small horse-shoe-shaped depression at its 
summit for the insertion of the arms. In one of my specimens (fig. 2. 6. and 7.) 
the lateral sides of the scapule bend in considerably, thereby producing an an- 
gular indentation, and giving the summit a stelliform figure. In this specimen 
may be traced the plates belonging to the integument that covers the abdo- 
minal cavity. 
9 
An Arn (fig. 1.) proceeds from each of the scapule, formed of four horse- 
shoe-shaped, and one cuneiform joint. 
