50 



larky of the column near the pelvis, we may readily admit a continuing new form- 

 ation of joints, by the muscular integuments developing and increasing in size. 



AUXILIARY SIDE ARMS. (PL. i.) In the late Mr. TOBIN'S specimen, 

 near the fractured end, at intervals of every 15th, 14th, or 12th joint, auxiliary 

 side arms proceed from each of the five spaces intervening between the petal- 

 like divisions; further up they come off from every 8th, 6th, and 4th, and near 

 the pelvis from every second joint. The intervening space between the petal- 

 like divisions being only near the centre calcareous, is peculiarly adapted to 

 supply the auxiliary side arms with nourishment, and to afford them points 

 of attachment, they have here also a more convenient space for action and 

 motion. Each of the columnar joints from which they proceed has five ovaU 

 transverse, and slightly concave impressions, surrounded by a rim sometimes 

 having a few radiating striae. Across the greatest diameter of the ellipticarea is a 

 ridge, having in its centre a perforation admitting the nourishing vessel, probably 

 supplied by vessels proceeding from the alimentary canal. Each auxiliary arm 

 (PL. ii. fig. 22.) when full grown is formed by about forty joints, of which those 

 near the column are more or less oval and thinner, the following becoming 

 thicker and rounder, but decreasing in size, their surface slants with an angle 

 of about twenty degrees from the perpendicular line of the column, admitting a 

 free motion upwards (on which side they also show a kind of fold) but op- 

 posing a movement downwards. The articulating surfaces (Pi.. H. tig. 15. to 

 17.) are surrounded by an elevated rirn, the interior is excavated into two cir- 

 cular concavities of unequal size, so disposed as to resemble the figure 8. the 

 two circles forming it, being separated by a small transverse ridge perforated by 

 the central passage of the nourishing vessel. (PL. n. fig. 23.) The termination 

 of the auxiliary side arms is a conic hooked joint, of a polished, greenish, en- 

 ameled appearance ; whereas the rest of the animal is of a pale yellow colour. 

 Near the summit of the column, about the fourth series of auxiliary side arms 

 from the pelvis, they become shorter and thinner ; here evidently showing 

 their greater muscularity, bend very slightly, and have their joints, particularly 

 those near the point (which as yet is not hooked) but faintly developed. (PL. 

 ii. fig. 21.) Those of the second series from the summit, are short, round, spine- 

 shaped, and particularly near the point so muscular, that not even the termi- 

 nating joint is discernible. (PL. 2. fig. 20) The first series of auxiliaiy side 

 arms (inserted on a joint which the lower protruding part of the first costals 

 almost touch) is still shorter, and almost resembles in appearance and thick- 

 ness a bristle. The formation of the lower side arms demonstrates the growth 



