OF WESTERN SIND. 189 



Family SPATANGIDjE. 



Subfamily SPATANGINJE. 



Genus MICKASTER, Agassiz, 1836. 



Test more or less heart-shaped and inflated, with the anterior margin indented 

 by the anteal sulcus. 



Apical disk compact, subcentral or excentric in front. 



The paired ambulacra petaloid, unequal, placed in shallow furrows. Poriferous 

 zones equal ; pores equal, pairs united by a conjugating furrow. The odd anterior 

 ambulacrum placed in a sulcus, which is usually broad but not very deep ; its pori- 

 ferous zones are very narrow, with minute pores placed in widely spaced, oblique 

 pairs. 



Peristome bilabiate, very excentric in front. 



Periproct oval, on the posterior extremity 



A single fasciole, subanal, encircling the inferior margin of the posterior ex- 

 tremity. 



Tubercles small, perforate and crenulate. 



1. MicEASTBE TUMiDus, Dimcan & 8laden. Plate XXXVII, Figs. 1-6. 



Test of moderate size, tumid, roundly cordiform, vertically truncate posteriorly. 

 Length rather greater than the breadth, or proportional as 1 : 0-95, the greatest 

 breadth being slightly in front of the centre. Anteriorly the marginal contour is full 

 and well-rounded, with the faintest suggestion of an impression corresponding with the 

 position of the odd anterior ambulacrum, and a slight flattening midway on the antero- 

 lateral interradia. Behind the line of greatest breadth the test converges gradually and 

 regularly, terminating with a rather sharp rounding on the posterior truncation. The 

 greatest height is excentric posteriorly, and is equal to about three fourths of the length, 

 the proportions of length to height beitig as 1 : 0"76. The test is high, full, and tumid. 

 When viewed in longitudinal profile, the anterior slope is seen to dip at a comparatively 

 slight angle, the anterior margin being very high, thick, and rounded. At the position 

 of greatest height there is a gentle convex curve, and the posterior slope is much more 

 rapid, as well as more convex, than the anterior. The posterior truncation is vertical, 

 and is well rounded beneath on to the actinal surface. Seen in transverse profile, the 

 great tumidity of the sides of the test is specially remarkable. The actinal surface is 

 convex, the resting-plane being along the median line of the plastron, whence the 

 test slopes downwards to the lateral margins. It is also rounded in fi-ont. There is a 

 slight prominence on either side where the posterior truncation unites with the 

 plastron. The periproct is very high, and placed at the top of the posterior extremity. 



The apical system is small, slightly excentric in front, and consequently far in front 

 of the general summit of the test; and its character is very compact. The four 



2c 



