from the Island of Malta. 369 



middle are somewhat longer, and the posterior single side is the 

 longest. There is scarcely any margin to the test in this species, 

 as the sides rise abruptly from the border to the apex, making 

 angles of 60^ with the base, and being only slightly curved in- 

 wards ; the dorsum is therefore very small in proportion to the 

 diameter of the base. The petaloidal portions of the ambulacral 

 areas extend over nearly three-fourths of the sides ; they form 

 long elegantly-shaped petals, narrower in proportion to their 

 length than those of C. alius, and consequently allowing of a 

 greater development of the interambulacral areas than in that 

 species; the petaloidal ambulacral areas are nearly all of the 

 same length, width and structure ; the centre of each petal is 

 arched and costated, and forms a considerable relief on the sur- 

 face of the test. The poriferous zones lie in slight depressions 

 on their sides ; each zone contains sixty-two pairs of pores set 

 widely apart ; in the inner row the holes are round, in the outer 

 row they are oblong, and both are united by straight oblique 

 sulci; the external surface of the partition-wall between each 

 pair is covered with a row of small tubercles. The bases of the 

 ambulacra are open, but not so widely as in some other con- 

 generic forms. The interambulacral areas are nearly flat, of 

 moderate width, and very uniform in their structure. The apical 

 disc occupies the centre of the dorsal surface ; it consists of two 

 circles of holes ; the outer is formed of five small genital holes, 

 the plates of which are not distinguishable, and the inner of five 

 small perforated ocular plates, which are distinctly visible at the 

 apices of the ambulacra. The madreporiform tubercle occupies 

 the centre, and forms a button-like prominence there ; the border 

 is thin and undulated, and this portion of the test presents a 

 striking difierence to the obtuse marginal fold seen in C. alius. 

 The base is quite flat ; the pentagonal mouth is small and nearly 

 central ; the oral lobes are curved inwards at an acute angle, and 

 the five ambulacral sulci are sharply defined as they radiate 

 from the sides of the pentagon to the border. The anus is 

 round, and is situated near the posterior border : the tubercles 

 on the upper surface are small and closely set together ; those 

 on the base are a little larger. 



Affinities and differences. — Clypeaster Reidii very much resem- 

 bles C. umbrella both as to height, width, and the smallness of 

 its tubercles ; it is distinguished from that species by the follow- 

 ing characters : C. Reidii is more elongated ; the petaloidal am- 

 bulacra are longer and narrower ; the interambulacra are flat- 

 tened and slightly curved ; the base is quite flat, and the oral 

 lobes are curved acutely inwards. In C. umbrella the interam- 

 bulacra are arched and costated, the base is concave, and the 

 oral lobes slope obliquely inwards. Compared with C. alius, the 



