OF WESTEEN SIND. 113 



having the periphery perforated by a circle of pores, lodged in small furrows, radiating 

 round the scrobicule. Tubercles crenulated and perforated. Spines compressed, 

 lamelliform, and strongly denticulated. Subsequently a recent Cidarid from the North 

 Atlantic was referred to this genus by Sir Wyville Thomson, notwithstanding that 

 the form appeared anomalous in being deficient in the very character upon which the 

 genus was originally founded, viz. the presence of pores in the scrobicule. The 

 radiating impressions, however, were present; and Sir Wyville Thomson remarked, 

 " It is perfectly conceivable that these depressions may sometimes actually go through 

 the test, or' at all events leave it so thin that the action of water and attrition during 

 the process of fossilization may be well supposed to have completed the perforation"*. 

 This latter remark appears to us highly sagacious and important. 



The first complete fossil test of Porocidaris known was that described by de Loriol 

 from the Nummulitic strata of Egyptf. He agrees with Thomson in the reference 

 of the recent form to the genus, and points out the interesting fact that both the pores 

 and the pits or grooves may be wanting on the same test — a circumstance probably 

 dependant upon their position thereon. It would hence appear that the scrobicular 

 pores and radiating grooves are of comparatively secondary importance. 



1. PoEOCiDARis ANOMALA, Duncan c& Sladen. Plate XXI, Figs. 10-14. 



Form subcircular, turban-shaped, depressed, the height being about one' half of 

 the diameter, moderately infiated at the ambitus, the contraction of the test" more 

 rapid actinally than abactinally. The test surrounding the actinostome is somewhat 

 depressed. 



Ambulacra straight, very broad, measuring two fifths of the interambulacral area 

 at the widest part, and contracting gradually towards the peristome, where it is half its 

 greatest breadth. The contraction towards the apical disk is not so great. Poriferous 

 zones very broad, scarcely sunken, half as wide as the interporiferous area. Pores wide 

 apart, the inner one slightly elliptical transversely, the outer one very decidedly so and 

 often contracting somewhat inwardly. The space between the pair of pores is channelled 

 by a slight groove, and a faint ridge may be traced in well-preserved plates passing 

 from the upper margin of the one pore to the lower margin of the companion. A 

 delicate ridge or keel running along the upper aboral margin of each plate separates 

 the neighbouring pairs of pores. There are eight pairs of pores opposite one of the 

 largest interambulacral plates at the ambitus. 



The interporiferous areas are broad, twice the width of the poriferous zone, 

 measuring 4 millim. in the largest specimen, and are furnished with six vertical rows 

 of minute mammillated miliary tubercles. There are consequently three tubercles 

 upon each plate ; the outermost is largest, and stands close to the inner pore and rather 

 nearer the aboral than the adoral margin of the plate ; the second is rather smaller, 

 and stands in the middle of the plate ; the innermost tubercle is much smaller, and is 



* Phil. Trans, vol. clxiv. p. 727. 



t ' Mouogr. des Echiaides contenus dans les couches Nummulitiques de I'Egypte,' 1881, p. 5. 



