HEMIASTER. 25 



of pores in each row. The antero-laterals are oblong-ovate, and have about twelve pairs of 

 pores in each row. The hollowed-out portion of the odd ambulacrum is ovato-lanceolate, 

 and longer than the antero-laterals by a third. The elevated spaces between the petals are 

 naiTow, and as if pinched up. The sides are very prominent. The caudal extremity 

 is perpendicularly truncate. The vent is small, and placed very high up. The ventral 

 surface is nearly plane, with rounded sides. The mouth is transversely oval, and scarcely 

 bilabiate. 



A large example measures half an inch in length by the same breadth, and four tenths 

 of an inch in height. As the test is not preserved in any specimen that I have seen, I have 

 not been able to make out the details of the plates, tubercles, and fasciole. 



Mr. Bowerbank has found many specimens in the London Clay of Sheppey. 



2. Hemiaster? Prestwichii. Plate III, fig. 5, a, b, c. 



Resembling the last in general aspect, but growing to a much larger size, and differing 

 in important particulars. It is very tumid, and suborbicular or slightly pentagonal in 

 outline. The postero-lateral interambulacral space is most elevated, and the back gradually 

 declines towards the frontal extremity. The amlaulacra are subpetaloid, and almost at the 

 siu-face of the test, the odd one only being lodged in a shallow depression. The antero- 

 lateral petals are two and a half times as long as the postero-laterals. The latter are 

 narrowly oblong, and in a moderate sized example present six pairs of pores in each row. 

 The antero-laterals are narrowly lanceolate or subparallel, and composed of about twelve 

 pairs of pores in a row. The caudal extremity is obtuse, and in its middle portion 

 perpendicularly truncate. The ventral surface is somewhat convex. The mouth is small. 

 The plates that are preserved are covered with shghtly scattered small equal tubercles, the 

 interspaces being granulated. I cannot detect any traces of fascioles, and am strongly 

 inclined to believe that this urchin is really a species of Macropneustes, but, until better 

 specimens are found, do not like to ventm-e on the introduction of that characteristically 

 Eocene genus into the British lists. 



Most of the specimens of this curious sea-urchin are deprived of all traces of their 

 tests. They have been found in the London Clay proper, especially at Sheppey, by 

 Mr. Bowerbank. The dimensions of a perfect, though not one of the largest examples, 

 are as follows : 



Length, Oy'ij-ths of an inch. Breadth the same. Height very nearly the same. 



3. Hemiaster Branderianus. Plate III, fig. 8. 



The remains of this rather large species are much compressed and broken down, 

 so that it is difficult to determine its original form. It appears to have been broadly 

 obcordate and elevated, though not over much posteriorly. It attains to three times the 



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