66 FOSSIL ASTEROIDEA. 



Dimensions. — The example figured on PI. XVI, fig. 3 a, 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 PI. XVI, fig. 5 a, was probably about the same size, judged by compu- 

 tation of the half-side. 



Localitij and Stratigrcq^hical 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 

 Asteriadee found fossil in British Sti'ata " (' Mem. Geol. Surv.,' vol. ii, p. 471, 

 184S), and figures of two examples were given in Dixon's ' Geology and Fossils of 

 the Tertiary and Cretaceous Formations of Sussex,' London, 1850, pi. xxi, 

 figs. 2, 2* ; pi. xxiii, fig. 15. The latter specimen is now preserved in the 

 British Museum (register-number " E 2585 "), and is illustrated on PL XVI, 

 fig. 5 a, 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. 



o 



Remarlcs. — Although at first sight the differences between Mitraster Hunteri 

 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. 3 a, 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 Mitraster 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. 3 a, the proportions 



