MITRASTER RLGATUS. 65 



The odd terminal or so-called " ocular " plate must be very small in order to 

 fit the space left by two adjacent ultimate supero-marginal plates ; but I am 

 unable to describe the plate, as I have only found a trace of it in one instance, and 

 have not seen a perfect one preserved in any of the examples of the species I have 

 examined. 



The abactinal area of the disk within the boundary of the marginal plates is 

 paved with large polygonal plates, whose surface is marked with a very minute, 

 closely crowded uniform granulation, upon which small miliary granules were 

 previously borne. Traces of these granules are still occasionally present in some 

 examples. The abactinal plates in this species are all large in relation to the size 

 of the disk, and the primary apical plates are much larger than any of the others. 

 The primary apical plates and several of the other plates have a central, small, 

 low, irregular eminence, such as might be formed by the merging together and 

 partial grinding down of a number of granules. I am unable to explain this 

 structure, or to consider it as associated with a pedicellarian apparatus, of which 

 I see no trace. It may possibly mark the remains of a tubercle which had 

 existed in an early stage of the animal's life, but had disappeared and been 

 outgrown at a later stage. 



The madreporiform body, which is very small, is circularly subtriangular in 

 shape, and is marked with fine regularly radiating striations. In the example 

 under notice two large plates surround two-thirds of the circumference of the 

 madreporite. In this specimen the madreporiform body measures IS mm. in 

 diameter, and the largest primary plate is 32 mm. in diameter. 



The infero-marginal plates are five in number, counting from the median 

 interradial line to the extremity, — that is to say, there are ten for the whole side of 

 the disk as against six in the supero-marginal series. The length of the two 

 innermost plates on each side of the median interradial line is greater than that of 

 the corresponding supero-marginal plates, consequently the second infero-marginal 

 plate extends a little way beneath the ultimate supero-marginal plate, and the re- 

 maining three infero-marginal plates are all under the ultimate supero-marginal 

 plate. In some examples I am inclined to think that probably only four infero- 

 marginal plates were present, counting from the median interradial line to the 

 extremity, in which case the edge of the second and the remaining two plates were 

 under the ultimate supero-marginal plate. 



I have unfortunately not seen any example of this species in which the actinal 

 surface of the disk is exposed, I am therefore unable to give the dimensions of the 

 infero-marginal plates on the actinal surface, or to describe their ornamentation. 

 For the same reason I am prevented from offering any remarks on the adam- 

 bulacral and actinal intermediate plates. 



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