THE SPINES. 655 



are remarkable for the great size and small number of the outer wedges of 

 the surface of the spines, and the narrow interlamellar cellular tissue, as, 

 for instance, in Echinus melo ; the spines of this family differ from those of 

 the Echinometradae proper by the small amount of interlamellar tissue, and 

 the smaller number of outer wedges. In some genera, as Mespilia and Trigo- 

 nocidaris, this feature is particularly striking ; yet in the genera Phymosoma 

 (PL XXXVI. f. n) and Hemipedina (PL XXXVI. f. u) the outer wedges 

 are minute, and the interlamellar space, owing to the indistinctness of the 

 concentric rows, recalls the structure of the interior of the shaft of the 

 Arbaciadae. In the genus Trigonocidaris the section of a primary interam- 

 bulacral spine (PL XXXVI. f. is) closely resembles that of the Diadema- 

 tidae in its structure ; in fact, the spines of Echinidae are like solid spines of 

 Diadematidae, with more marked concentric rings of growth. 



Tbe spines of the Echinonidae (PL XIV. f. 2) resemble more those of the 

 Desmosticha than those of the Petalosticha ; the same is also the case with the 

 spines of the abactinal region of Echinolampas (PL XVI. f. "13, 16). The spines 

 of Rhynchopygus (PL XXXVII. f. i„>) do not differ from those of the other 

 Petalosticha. In the Clypeastridae and Petalosticha the spines are remark- 

 able for the proportionally great size of the collar and the prominence of the 

 milled ring (PL XXXVII f. 3, 16, 18, 20) ; also for the peculiar articulation 

 of the condyle, which is not smooth as in the other Echini, but consists 

 of four or more knobs fitting into the crenulations of the mammary boss 

 (PL XXXVII. f. 3, 4, 9, ll). Owing to the great size of the collar compared 

 to the length of the spines, they are capable of rapid movements. This is 

 readily observed in living specimens of Echinarachnius, Mellita, Rhynchopy- 

 gus, Moira, Brissus, Meoma, showing probably that these spines are far more 

 sensitive than the spines of the Desmosticha, and may in these instances be 

 even coarse organs of touch. The spines of the actinal and abactinal sides 

 of the Clypeastroids are usually different ; they are clavate and short on the 

 abactinal side (see Mellita, PL XXXVII. f. 1); while they are curved, 

 pointed, and slender on the actinal side (PL XXXVII f. 2). The same is 

 the case in Echinodiscus (PL XXXVII. f. .4). In the Laganidae (PL 

 XXXVII f. 3) and Echinanthidae (PL XXXVII f. 9), there appears to 

 be no difference in the spines of the two surfaces. 



In the Petalosticha the spines of the abactinal surface are curved above 

 the milled ring (PL XXXVII. f. 14, 16, 19), and pointed, gradually tapering 

 towards the tip ; while the spines of the actinal surface, more especially those 



