ECHINOTHURIDiE. 143 



are rarely verticillate as in Diadema. They are commonly hollow, though 

 some of the very slender ones and those on the buccal membrane are oft en 

 solid, at least in their distal half. The sheathed spines are commonly 

 primary spines, but may be secondaries or even miliaries; when the 

 sheath of skin is thick and loose they are conspicuous and unmistakable, 

 but when the sheath is thin and close-fitting they can scarcely be dis- 

 tinguished from ordinary "naked" spines, which are, of course, really 

 covered by the skin. Sheathed spines may be either actinal or abactinal 

 in position, and the sheath may be either cylindrical or distally swollen ; 

 when cylindrical, it may have one or more circular constrictions. The 

 glandular spines are secondaries or miliaries, exceedingly sharp, and pro- 

 vided with a poison-gland at the tip. They are commonly abactinal in 

 position, and may be arranged in regular series on the interambulacra. The 

 hoofed spines are always actinal primaries, and are the most conspicuous 

 spines on the test when present. They are usually hollow, even the shining 

 white hoof-shaped swelling in which they terminate being only loosely 

 filled with calcareous strands ; but in some species the hoof and the distal 

 half of the spine are perfectly solid ; there is more or less diversity how- 

 ever in different spines, even of a single individual. Unfortunately the 

 hoofs are very easily broken off, and otherwise good specimens may show 

 few or none. All four of these different kinds of spines may occur on 

 a single individual, but as Mortensen has pointed out, species which have 

 sheathed spines actinally do not have hoofed spines. 



The pedicellarice of the Echinothuridas have been so fully discussed and 

 satisfactorily figured by Mortensen and Doderlein, it is hardly necessary 

 to devote much space to their description. There are four different sorts, 

 two or more of which may be found in any individual. The triphyllous 

 pedicellaria? (PI. C4, fig. 3) are always present, and always have three valves, 

 which are broadest at or near the tip and are well separated from each 

 other just below the middle when the pedicellaria is closed ; the valves 

 (PI. 64, figs. S, 12) are provided with a " cover-plate " which is usually more 

 or less perforated. These pedicellaritB are abundant on almost all parts of 

 the test, and are borne on long slender stalks, which are several times as 

 long as the head. The tridentate pedicellaria? (Pis. 64, fig. 9 ; 66, figs. 1, 2, 

 15, 17; 67, figs. 2, 3, 4, 6, S, 9, 12, 17) are nearly or quite as common as the 

 triphyllous, but are very much more diverse in size and form. They occur 

 on all parts of the test, but their abundance varies greatly in different indi- 



