


24 
secure adhesion to extraneous bodies. We may thence conclude that here the 
office of this laminar ring-like concretion, was to repair an injury sustained 
by the column. 
The SUPERIOR ENLARGED COLUMNAR JOINT (T. Iv. fig. 13. and 19.) resem- 
bles in its inferior surface those preceding it. but as its centre commands the 
entrance to the alimentary canal, it is provided with a sphincter muscle capa- 
ble of close contraction, as is proved by a specimen, perhaps unique, in my 
possession, (T. rv. fig. 13. to 15.) where the folds produced by the action of 
this muscle are distinctly seen. Its upper surface has five ridges diverging 
from the centre, the space between which is, more or less, concave. The cir- 
cumference of the superior margin is still somewhat greater than that of the 
inferior, and the outer surface of the joint swells out in a trifling degree towards 
the middle, being slightly contracted towards either margin. Sometimes the 
upper surface of this joint is peculiarly convex (T. rv. fig. 16. and 18.) in which 
case, generally, both these margins appear much compressed ; this is evidently 
a result of contraction, and an additional proof of the original unindurated state 
of the exterior part of the joint. 
Before we proceed to examine in detail the separate pieces composing 
the body, and completing together with the upper enlarged columnar joints 
just described, the pyriform shape that characterises the superior portion of 
this animal, it will add much to the clearness of the ensuing description’, to 
state in a concise manner, although at the expense of some repetition, the ge- 
neral form and relations of this part and its constituent members. (See the 
plate illustrating generic characters). n 
In each of the four rows which succeed the upper enlarged columnar joint, 
_ the circumference of the body becomes divided into five distinct and similar 
joints; which although necessarily undergoing much subordinate variation of : 
figure, to fit them for their respective places and combinations, have yet many 
general points of analogy in all these rows, They all have a wedge-shaped, or 
rather truncated pyramidal form, being arranged round the central cavity of the. 
body, like the blocks of masonry which compose the courses of a cupola, present- 
ing their broader ends or bases towards the exterior circumference, their nar- 
‘row end or truncated apex towards the interior. Hence, in describing these 
joints, we have always six surfaces to take into consideration; the EXTERIOR 


iks 
