I LLAMOCEIN! - DIOMED l 



The syzygial sutures arc already in the young stages recognizable as 

 such, and presenl marked differences in the outlines with the sutures which 

 arc to bo muscular sutures. Compare the young syzygy, Plate XII, Fi 

 with the other young joints figured in Plate XII. Figs. 15, 16. A terminal 

 arm joint is figured in Plate XII. Fig. 13 j it presents as yel no indication 

 of forking to form the pinnule; hut from in length it is probable that some 

 of the larger shanks of the terminal cell- will soon develop more rapidly 

 than the rest, ami that a new joint will then he formed with a rudimentary 

 articulation such as is figured on Plate XII. Figs. 15, hi. The last two 

 joints of Figure II of the same plate carry no pinnules, so that one of 

 the newly formed sutures will probably be a syzygy. 



Pinnules. 



There is a very marked difference in the aspect of the pinnules of differ- 

 ent parts of the arms. Those pinnules which are below the axillary, the 

 greater part of which, or the base at least, is closely connected with the 

 perisome, are characterized by the great solidity and shortness of the joints, 

 and the powerful wing-like expansions of the lower part of the joints (Plate 

 XYI. Fig. 1). Those on the main branch of the stem, from the first axillary 

 to the last, are somewhat longer, with more elongated joints, and the wings 

 at the base of the joints narrower and more distinctly separated (Plate 

 XVI. Figs. 2, 3, 5). Those of the forks and of the extremity of the arm 

 are marked by a gradual decrease in the size of the wings, until they 

 become, like the pinnules near the end of the main arm i Plate XVI. 

 Fig. 4), rather slender, with but slight lateral projections at the base of 

 each joint. 



The distal pinnule joints resemble in their structure that of the younger 

 arm joints before they have developed their characteristic sutural faces, with 

 the exception that on the ventral edge of the pinnule joints towards the 

 base there develops on either side, or one side alone, a sort of buttress-like 

 wing, which becomes stronger as we approach the basal joints of the pinnule. 

 See Plate XVI. Figs. 6-13. with corresponding views of the young arm 

 joints figured in Plate XIII. Figs 3-9. The arrangement of the reticulation 

 of the ventral surface beyond the floor forming the axial canal in radiating 

 lines is, however, usually more marked in pinnule joints than in arm joints 

 (compare Plate XVI. Fig. 8, with Plate XII. Figs. 8, 11); although in other 

 arm joints (Plate XIII. Figs. 4-6) the arrangement of the reticulation in 



