OF WESTERN SIND. 347 



no secondary tubercles resembling those of the interradia on the ambulacral surface, 

 the comparative plainness of which contrasts with the regular and large ornamentation 

 of the interradia. 



Within the groove at the margin of the test in front, the ornamentation of the 

 ambulacrum is small and closely granular, and there is an indefinite series of very 

 small but perfect tubercles, which may be traced to the apical region. The position of 

 the somewhat broken series is just between the pairs of pores and the median line. 

 These little tubercles become larger actinally and wider apart. Finally the actinal 

 surface of this ambulacrum, although it appears flat, is slightly convex from side to side, 

 and there is no median grooving ; but the peristomial plates rise in the proper position 

 of the test, and form part of the anterior margin of the peristome. 



The antero-lateral ambulacra are nearly transverse in those parts which are ex- 

 ternal to the internal fascicle, especially the anterior poriferous zone. This may be 

 slightly curved, so as to bring the outer end more to the front, or there may be a double 

 curving, which renders the zone rather sinuous, there being a convexity forwards in the 

 neighbourhood of the fasciole. 



The posterior zone is wide apart from the anterior close to the internal fasciole, 

 and either there is a nearly straight part between this part and the external end of the 

 ambulacrum, or there is some slight sinuosity of the posterior zone. There is much 

 variation in the relative direction of these two zones, and in some large specimens there 

 is an anterior concavity of the anterior one. 



The ambulacra now under consideration are angular at the external end of the 

 perfect poriferous zones ; within the internal fasciole the ambulacrum is also triangular, 

 the base being at the fasciole and the apex at the radial plate. In this triangle the 

 posterior zone of pores is nearly transverse, whilst the anterior zone slants from the pair 

 of pores immediately in contact with the internal fasciole to the proper radial plate. 



The poriferous zones outside the internal fasciole are broad, not very long, for 

 they do not pass over the sudden curve of the test above the ambitus, and the pairs of 

 pores are in deep and large depressions which are separated by costae, thin and granular 

 on the free edge, and thick and very solid where they merge into the floor of the 

 poriferous plate. 



The pores of each pair are large, wide apart, deeply seated, and the anterior row 

 of the anterior zone and the posterior row of the posterior zone are larger than 

 those of the other rows. The smaller pores are nearly circular in outline, and the 

 largest are pyriform. The whole of the ambulacrum external to the internal fasciole 

 is slightly depressed below the level of the interradia, but whilst the poriferous zones 

 are in shallow grooves the interporiferous areas are rather tumid. These last are crowded 

 with granules of at least two kinds, and some very small tubercles are found amongst 

 them with tiny mamelons. Near the internal fasciole the granules and minute tubercles 

 are arranged in more or less perfect rows parallel to the fore and aft running fasciole. 



The pair of pores in the anterior zone next to the fasciole abort more or less ; but 

 they are really miniature representations of the larger. In many large specimens, how- 



3a2 



