66 FOSSIL ASTEROIDEA. 
Dimensions.—The example figured on Pl. XVI, fig. 3a, has a major radius of 
about 17°5 mm., and the minor radius is about 13°75 mm. The length of the side 
is about 20 mm. The fragmentary type-specimen figured by Forbes, which is 
drawn on Pl. XVI, fig. 5a, was probably about the same size, judged by compu- 
tation of the half-side. 
Locality and Stratigraphical Position.—One of the type specimens figured by 
Forbes is stated to have been obtained from the Upper Chalk of Wiltshire, but 
the second specimen, which is now preserved in the British Museum, bears no 
record of any locality. Forbes also records the species from Kent and Sussex. 
Authentic examples from the Upper Chalk from Gravesend and ‘ Kent” are 
preserved in the British Museum. 
History.—This species was described by Forbes in his memoir ‘On the 
Asteriade found fossil in British Strata” (‘Mem. Geol. Surv.,’ vol. ii, p. 471, 
1848), and figures of two examples were given in Dixon’s ‘ Geology and Fossils of 
the Tertiary and Cretaceous Formations of Sussex,’ London, 1850, pl. xxi, 
figs. 2, 2*; pl. xxii, fig. 15. The latter specimen is now preserved in the 
British Museum (register-number ‘‘ H 2585”), and is illustrated on Pl. XVI, 
fic. 5a, of the present work. I have not been able to find any trace of the other 
example figured by Forbes, which originally formed part of the collection belonging 
to the late Mr. Channing Pearce. Forbes states that it was found in Wiltshire. 
Remarks.—Although at first sight the differences between Mitraster Huntert 
and Mitraster rugatus appear well marked, I am not perfectly satisfied as to the 
species being altogether independent. When the types alone are examined there 
appears to be no need for any doubt upon this question. But examples occur 
which are exceedingly difficult to determine on account of presenting features 
which seem to break down some character which has been regarded as diagnostic 
of the other species. In illustration of this difficulty I have drawn on Pl. XVI, 
fig. 3a, an example which I have ranked under Mitraster rugatus, but which 
presents considerable superficial resemblance to Mitraster Hunteri in the character 
of the ornamentation of the supero-marginal plates. I believe, however, that the 
proportions of the plates, the absence of any abactinal gibbosity, and the exten- 
sion of the tuberculation over the whole abactinal area constitute, inter alia, a 
justification for regarding the example as Mitrasier rugatus. 
Turning, on the other hand, to a series of Mitraster Hunteri, considerable 
variation is to be noted in the relative length and breadth of the supero-marginal 
plates, as well as in the amount of gibbosity developed on the abactinal area of 
the plate. In such an example as that figured on Pl. XV, fig. 3a, the proportions 
