274 MICRASTER 



The plates in the ambulacral areas likewise support tubercles, but they are smaller 

 and more sparsely distributed than in the inter-ambulacra ; the relative magnitudes of 

 the tubercles in the two areas is shown in fig. 1 /, where they are both drawn to a scale 

 of four diameters. The tubercles on the sides and upper surface consist of a perforated 

 tubercle, placed upon a crenulated boss, and surrounded by a smooth areolar space, not 

 scrobiculated on the upper surface, but very distinctly so at the base. They become 

 much more numerous around the ambitus, and increase in size on the basal plates of the 

 postero-lateral inter- ambulacral areas, fig. 1 b. On the triangular central space extending 

 from the fasciole to the under lip, the tubercles are arranged in well-formed rows that radiate 

 forwards and outwards from a central tubercle and fill the entire space with close-set, 

 regularly arranged tubercles, as shown in fig. 1 b \ the structure of the tubercles is very 

 well delineated in the enlarged drawing of the same, made from a portion of the base where 

 each areolar space is seen to be surrounded by a circle of granules and the intermediate 

 portion filled in with the same. 



The tubercles supported small, needle-shaped spines, some of which are preserved on 

 one of our specimens ; of these spines a few are slightly bent near the base, fig. 5 ; all are 

 marked by fine longitudinal ridges, fig. 3 (destitute of spiral ornaments), which disappear 

 towards the apex; at the acetabulum a crenulated ring is visible. The minute spines 

 with distinct ridges, which extend from base to apex, and which are traversed by a series 

 of spiral depressions, fig. 4, occasionally found in the beds containing M. cor-anguinum, 

 belong to Echinothuria floris and not to M. cor-anguinum. 



The apical disc is small and excentral, and composed of four perforated ovarial plates, 

 the right antero-lateral being much the largest and extending into the middle of the disc ; 

 it is covered with the madreporiform body. The five ocular plates are wedged into the 

 angular spaces left by the ovarials ; the discal elements being firmly soldered together, this 

 portion of the test is nearly always well preserved. 



The posterior border is flat, abruptly truncated, and obliquely inclined downwards 

 and forwards. The vent occupies the upper part of this region, and opens immediately 

 under the rostrated carina. A few small tubercles are sparsely disposed below the 

 vent and the lower part of the space is smooth, the fasciole encircles the angle, one 

 half passing above, the other half below the basal angle. It is well seen in most 

 specimens, and I have given a drawing of it in fig. 1 k, where it is seen passing as a 

 finely granulated band between the tubercles on the plates. 



The mouth is placed very near the border, at the inferior termination of the deeply 

 impressed anterior ambulacrum. It varies in position slightly in different specimens. 

 The lower lip is thick and strong, and varies in the amount of projection, fig. 1 b and e. 



Affinities and Differences. — This species, so long known and so often described, has 

 nevertheless been the subject of much confusion. The older authors, as Lister and 

 Llhwyd, appear to have been acquainted with its moulds in flint. Breynius and Klein 

 studied its test, and the latter described it in 1734 under the name Spatangus cor-anguinum. 



