84 



nites, and hence the assertion that, species of that genus occur in mountain 

 and transition lime-stone. But the absence of the pentapetalous serm'striated 

 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 COLUMNAR 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 canal, 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 PELVIS (fig. 3.) formed of three plates is cup-shaped, sustaining five 

 long scapulae (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 scapulae 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. 



An ARM (fig. 1.) proceeds from each of the scapulas, formed of four horse- 

 shoe-shaped, and one cuneiform joint. 



