FROM THE LOWER GREENS AND. 151 



there are six plates in eacli column ; the tubercles increase in size from the peristome 

 upwards, so that the three upper pairs of tubercles are the largest and most prominent 

 of the series (fig. 1 d and fig. 1/). A few large mammillated granules, unequal in size, 

 are arranged round the areola and in the miliary zone ; a double row of smaller granules 

 occupy the middle space ; the tubercles are raised upon large bosses with deeply crenu- 

 lated summits, and the mammillon is prominent, projecting, and imperforate (fig. 1/). 



The apical disc is very large, convex, subcircular, and entirely smooth, presenting in 

 the line of its sutures two or three isolated superficial punctations in some of them 

 (fig. 1 b) ; the sur-anal plate occupies the centre of the disc, and is situated immediately 

 before the vent ; in one example (fig. 2) however, it is abnormal and rudimentary, and 

 forms an exceptional variety in an otherwise remarkably regular and symmetrical structure. 

 Of the many specimens of this Urchin that have passed through my hands this is the only 

 exception that has come under my notice. All the ovarial plates are perforated near the 

 centres of each ; the madreporic laceration is small, but very distinct in the rio-ht 

 antero-lateral ovarial (fig. 1 h). The periprocte is sub-elliptical, rounded anteriorly, an o-ular 

 posteriorly, and somewhat elevated and inflated at the border (fig. 1 b and fig. 2). 



The base is flat and the mouth-opening large, one half the diameter of the test (fio-. 1 c). 

 The peristome is circular and divided by feeble incisions into ten nearly equal-sized 

 lobes. 



Affinities and Differences. — This species presents great uniformity in its specific 

 characters, after a careful examination of a great many specimens I have found very little 

 variation from the type form I have figured and described. This specimen, belono-ino- to 

 the British Museum, is unexceptionally large, and perfect in all its parts. Smaller 

 specimens are, however, equally good for description. Tlue Sponge-gravel in which this 

 Urchin is most commonly found is in general a very bad matrix for the preservation of the 

 tests of Echinidse ; when shielded by a ferruginous crust, derived from the matrix, the 

 more delicate portions of the urchin are wonderfully preserved. P. Wrightii resembles 

 P. stellulaius ; it is separated from that form by having a greater number of less promi- 

 nent tubercles in the area ; the apical disc is likewise thinner and smoother, and the sutural 

 punctations are more isolated and more feebly marked. 



Locality and Stratigraphical Position. — I have collected Peltastes Wrightii from the 

 Sponge-gravel bed of the Lower Greensand near Farringdon, Berks; it has been found 

 likewise in the Lower Greensand at Hythe, Kent; and at Atherfield and Sandown 

 Isle of Wight ; it is, therefore, a true Neocomian or Lower Greensand form. 



History. — This species is a very distinct form, and has been separated by Professor 

 Desor from P.pimctatus, with which it had been identified by most of our English authors. 

 The accuracy of this determination has been sanctioned by M. Cotteau, who has given a 

 very good description of the species, with excellent figures thereof, in his great work on 

 the Cretaceous Echinida3, in the ' Paleontologie Frangaise.' 



