THE SPINES 



As is well known, the tubercles of Echini, from the few large primary 

 interambulacral tubercles of the Cidaridae to the minute tubercles of the 

 abactinal part of the test of the Spatangidae, carry spines which in their 

 general features are identical. Whether we examine the large solid spines 

 of the former or the silk-like spines of the latter, they consist of a shaft and 

 of an articulating head or condyle, forming a ball and socket joint with the 

 tubercle proper (PI. XXXV. f. 1, 4, 8, 10; PI. XXXVI f. 3, 5, 6, 7, 10, 14; PI. 

 XXXVII.) ; the ligaments which keep the spine in place conceal its base, 

 and are attached to the outside edge of the scrobicular circle and to the 

 milled ring or collar which is at the base of the shaft proper. The notches 

 of the milled ring are the continuation of the stria? or ornamentation of the 

 shaft proper, only the calcareous tissue is more compact, and the dividing lines 

 between the longitudinal striae are so marked as to be readily distinguished 

 with the naked eye as indentations in the circumference of the flaring collar. 



The longitudinal and transverse sections of the principal families given on 

 Pis. XXXV. - XXXVII, show the more important features distinguishing 

 them. We And, as far as the structure is concerned, greater differences be- 

 tween the Cidaridae and the other Desmosticha than we find between the 

 Clypeastroids and Spatangoids on one side and the remaining Desmosticha 

 on the other. Yet, to judge from the external appearance of the small silk- 

 like spines of the Petalosticha and Clypeastroids when compared with the 

 comparatively huge spines of the Desmosticha, we might have expected 

 marked structural differences. 



In the Cidaridae the whole of the shaft is made up of small limestone 

 meshes, radiating from a central part, consisting of somewhat larger cells. 

 These cells are simply arranged in spokes, and do not greatly increase in size 

 towards the periphery (PL XXXV. f. 2-0). The exterior of the shaft is 

 made up of larger elliptical limestone wedges, shooting out irregularly from 

 the periphery of the homogeneous interior, and connected by irregular cross- 

 bars, which form the spines, serrations, granular and other ornamentation of 

 the spines of Cidaridae. As fast as the outside sheath increases in breadth, 

 by sending out shoots from the outer edge, and by the increase in number 



