168 Dr. G. J. Hinde on a 



to be worn down to the surface of the plate, so that in many- 

 instances they look like so many minute rings, and their 

 real characters would scarcely be recognized in this condition. 



The spines are elongate, cylindrical in section, with a 

 slightly prominent ring or collar at their basal extremity 

 (PI. VI. fig. 8). The base is perforated with a minute cir- 

 cular pit. Just above the basal collar is a slight contraction 

 or neck, beyond which the spine gradually increases in thick- 

 ness to near its middle and then tapers to an obtuse or, in 

 some cases, a sharp point. In some examples the spine is 

 thickest near the base, the collar is not perceptible, and it 

 tapers regularly to its distal end. The surface of the spines, 

 when examined under the microscope, is seen to be orna- 

 mented with minute longitudinal parallel ridges, which do 

 not appear to be continuously even with its surface, but at 

 intervals project outward at a small angle ; these ridges are 

 about *025 millim. apart. The spines vary from 1*5 to 

 4 millim. in length, and from *25 to "35 millim. in thickness. 

 Those on the basals and first radials are tolerably uniform in 

 size, and average 2"5 millim. in length, whilst those of the 

 vault-plates are only 1'5 millim. in length, but nearly as 

 thick as those of the calyx (PI. VI. fig. 9). The spines on 

 the arms appear to be very slender. 



The excavated basal faces of the spines are approximately 

 of the same dimensions as the summits of the tubercles, and 

 could therefore but very slightly have overlapped or clasped 

 these latter, and they would consequently be mainly held in 

 position by the ligament passing between the cavity in the 

 tubercle and that in the spine. 



There is no apparent regular distribution of the tubercles 

 and spines on the plates of the calyx. In some few cases 

 there are rows of tubercles which seem to be parallel with the 

 margins of the first radials ; but, as a rule, the tubercles are 

 irregularly scattered over the plates, sometimes being in close 

 proximity or only about 2 millim. apart, whilst not unfre- 

 quently there is an interval of 1 millim. between two of them. 

 They are most thickly grouped on the upper tumid portion 

 of the first radial, just below the excavated surface on which 

 the second radial rests. There are as many as seventy-three 

 spine-bearing tubercles on the surface of a large first-radial 

 plate, and from fifty to sixty on smaller plates. A large basal 

 plate has sixty- five tubercles, whilst an unusually small plate 

 possesses only twenty-seven. The tubercles and spines of 

 the radial and basal plates are approximately equal in size, 

 but there are two distinct sizes of tubercles on the smaller 

 vault-plates. The interspaces on the plates between the 



