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 striae, 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 PELVIS ; those of the second, the FIRST COSTAL; those of the 

 third, the SECOND COSTAL ; and those of the fourth, the SCAPI/LJE. 



Above the scapulae 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 FINGER 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 PLATES i. and m. 



The PELVIS (Pt. v. fig. 1. to 6. and 19.) is formed by five wedge-shaped or 

 truncated pyramidal plates, each resting by its inferior surface, which is con- 

 cave, upon one of the five concave spaces of the superior enlarged columnar 



E 



