OF WESTERN SIND. 349 



opposite ambulacrum no. II., the peristomial plate of zone a is the largest, and has two 

 pores, each surrounded by a peripodium, whilst the smaller plate of h has but one, and 

 there is a cavity in which a sphseridium was probably once placed. The ornamentation of 

 the peristomial part of the ambulacra resembles that of the odd one ; but in passing over 

 the side of the test and on to the actinal surface to the third plate from the peristome, the 

 plates have the ornamentation of the interradia. The peristomial edge of these ambu- 

 lacra is a narroM', curved, plain ledge. There are usually one or two peripodial pro- 

 cesses beyond those of the peristomial plates, and it is interesting to note, in some 

 instances, that these peripodia occupy the place of a tubercle. 



The posterior ambulacra are longer than the antero-lateral, are rather close, reach 

 to the part of the test where narrowing commences, and have an extra- and an intra- 

 fasciolar part. 



Beyond the fasciole, the outer poriferous zones are long, nearly straight, being 

 slightly bent near it. Like all the zones, this one is slightly sunken. The general 

 construction is that of the zones already described, and the number of pairs of pores is 

 from twelve to twenty, according to the length of the individual. 



The inner zone, beyond the internal fasciole, is shorter than the outer, commences 

 on a line much behind it, is slightly wavy, and comes close to the outer zone so as to 

 close the petal. The interporiferous area is slightly tumid, and ornamented as the 

 antero-lateral area. Close to the internal fasciole the pairs of pores are smaller than 

 elsewhere in that part of the zone, and, except in very large specimens, there are no 

 large pairs within the fasciole. 



"Within the fasciole the direction of the pairs of pores of both of the zones 

 changes, and some pairs of very minute pores pass towards the radial plate, with a 

 direction forwards and inwards from the outer zone. They are in a slight groove. 

 The internal poriferous zone within the fasciole is longer than the outer, its direction 

 is almost parallel with the median line behind the apical disk, and it is separated from 

 its fellow by the narrow odd interradium. 



The actinal part of these ambulacra is very broad on the side of the sternum, and 

 their plates are unusually large and distinct. 



At the peristome the width of the zones is less than further back ; but it is great 

 nevertheless, and the two sets of peristomial plates of the ambulacra V. and I. are so 

 large that they contrast with the narrow posterior lip, and the still narrower peristomial 

 ends of the interradia on either side. The plates at the peristomial margin of zones 

 a of ambulacrum I., and h of ambulacrum V., are larger than the plates of the zones 

 h and a of ambulacra I. and V. respectively. 



'J'he larger plates, moreover, are pierced by two pairs of pores and have two peri- 

 podia on each. The outer and smaller plates have only one peripodial process. There 

 are deep depressions close to the peripodia for the sphseridia in all the peristomial 

 plates of these ambulacra of the bivium. With regard to the other plates of the 

 posterior ambulacra, they are symmetrically placed on either side of the sternum, and it 

 will therefore only be necessary to treat of those of one ambulacrum. Taking no. 1, 



