25 
CIRCUMFERENCE, the INNER TRUNCATED EDGE, the UNDER SURFACE by which 
they rest on the row beneath them, the UPPER suRFACE which they present 
towards the row above them, and the two LATERAL SURFACES which they pre- 
sent to the contiguous joints of the same row. These terms will be henceforth 
used, without further definition, in describing each of these joints. 
In each successive row, the inner surface recedes further and further from 
the centre of the body, and opens more and more widely the internal cavity 
which contained the mouth and viscera. 
Each joint has near its inner edge various tubercular (sometimes TREFOIL 
SHAPED) PROCESSES, with perforations destined apparently for the insertion and 
conveyance of muscles and vessels, behind which there is either a remarkable 
SUBTRIANGULAR RIDGE across the upper or lower surface, or a tubercle with an 
elongated muscle. ‘The outer surface is always plain; the upper, under, and 
lateral sides are always marked with radiating striz, interrupted by transverse 
concentric ridges, in a Manner exactly similar to those which have been already 
described in speaking of the columnar joint, and forming a continuation of the 
same system, 
The joints of the first row, will, from the analogy of their places and offices, 
be termed the Peiyis; those of the second, the First CosraL; those of the 
third, the Seconp Costa.; and those of the fourth, the ScapuLz. 
Above the scapule a further subdivision of the circumference into ten dis- 
tinct pieces takes place. The two first rows of this subdivided series may be 
considered as Arm Joints ; the succeeding rows as Fincer Joints, which send 
off smaller jointed tentacula, one from every finger joint. 
A correct representation of all these pieces, sufficiently detached to show 
their general form, but still arranged in their relative position, will be found in 
the plate illustrating the character of the genus; each piece will be seen se- 
parately figured in PLates v. and vi. and external and internal views of the 
united body as composed of them, in PLares 1, and 111. 
The Pexvis (Pt. v. fig. 1. to6. and 19.) is formed by five wedge-shaped or 
truncated pyramidal plates, each resting by its inferior surface, which is con- 
eave, upon one of the five concave spaces of the superior enlarged columnar 
E : 
