from the Island of Malta. 83 



disc* : the posterior poriferous zone extends two-thirds of the 

 distance between the disc and the border; the anterior zone 

 commences much farther from the disc above, but extends as far 

 as the posterior zone below; thus in the specimen before us, 

 there are fourteen pairs of pores in the posterior, and only nine 

 in the anterior zone : the postero-lateral ambulacral areas ex- 

 tend backwards, and towards their termination they curve grace- 

 fully outwards ; the angle they form at the transverse line at the 

 disc is 62, and there are twenty pairs of pores in each of the 

 zones. Each of the interambulacral areas presents peculiar 

 characters ; the antero-lateral pair are the smallest, and the pos- 

 tero-lateral are the largest; they are both remarkable for the 

 tubercles they support on their dorsal surface ; in the anterior pair 

 there are nine, and in the posterior pair there are fourteen of these 

 tubercles on each side of the test, which are sculptured out of 

 the substance of the plates ; as they do not project beyond the 

 general surface of the shell, each tubercle is seated on a cylinder, 

 which is surrounded by a deep, wide depression : as these perfo- 

 rations are not arranged with much regularity, the test has the 

 appearance of having been bored by some marine mollusk. 

 The tubercles themselves, in proportion to the size of the sup- 

 porting cylinder and encircling entrenchment, are small and per- 

 forated; the single interambulacrum is narrow and elongated, 

 and rises in the mesial line into a prominent elevated ridge ; the 

 posterior border is obliquely truncated downwards and inwards, 

 in the upper part of this space the large anal opening is situated ; 

 below, the interambulacrum forms another prominence marked by 

 two concave depressions on each side thereof, from which a wide, 

 gently sloping central space occupies the middle of the ventral sur- 

 face, having the large transversely-oblong mouth-opening with its 

 projecting posterior lip occupying the anterior third of the base ; 

 on the sides of this sloping central space the basal portions of 

 the postero-lateral interambulacra are thickly studded with 

 large slanting perforated tubercles, arranged with much regu- 

 larity in this region, and a few smaller tubercles are scattered 

 over the basal portion of the antero-lateral pair ; the crescentic 

 depressions below the anus have each a group of perforated 

 tubercles, and between them and the smooth central concave 

 portion of the base is the subanal fascicle, which forms a trans- 

 verse band in the middle, and a crescent on each side, the upper 



* It may be as well to state, that we have adopted this mode of mea- 

 suring the amount of divergence of the antero-lateral and postero-lateral 

 areas rather than the vague expressions formerly in use. The angle is 

 measured from a transverse line which cuts the longitudinal axis at right 

 angles at the apical disc ; we have found the degree of divergence to be 

 very uniform in each species. 



