36 FOSSIL ASTEROIDEA. 



fossils so named by him were given in Dixon's ' Geology and Fossils of the 



Tertiary and Cretaceous Formations of Sussex,' London, 1850. One, if not 



more, of the specimens delineated in that work is now preserved in the British 

 Museum. 



Variations. — In some examples the breadth of the border formed by the 

 supero-marginal plates on the abactinal surface is greater in relation to the disk- 

 area than in others, and this variety was noted by Forbes (' Mem. Geol. Surv.,' 

 vol. ii, p. 472). I have not been able to establish the relation of this modification 

 with any other permanent morphological character, nor to associate it with any 

 special locality or stratum, and I am therefore led to consider, for the present at 

 least, that the variation in question is one affecting individual examples of the 

 species independently of other structural modifications which would warrant 

 recognition by name. 



Two other variations are to be noted in this species which are superficially 

 much more striking, and either of them would, if only isolated examples were 

 known, lend a strong temptation to the separation of their possessor from the 

 normal form of the species. One of the variations in question affects the large 

 ultimate paired supero-marginal plates. On comparing the examples drawn on 

 PI. X, fig. 1 and fig. 2 a, with fig. 1 a, PI. XII, and fig. 1 a, PI. XI, it will be seen 

 that the ultimate plates are relatively much larger than the adjacent supero- 

 marginal plates and are more produced at the extremity ; whilst in the specimen 

 delineated in fig. 2 a, PI. XI, this modification is carried to such an extent that 

 at first sight it would appear scarcely possible to believe that this fossil belongs 

 to the same species as, for example, fig. 2 a, PI. X. I have, however, been unable 

 to find any other constant variation from what has been considered the typical 

 form of Metopaster Parkinsoni associated with this modification in the size and 

 shape of the ultimate plates ; and as the most complete gradation between the two 

 extremes may be traced in the splendid series of specimens now preserved in the 

 British Museum, all obtained from the same locality and the same horizon, no 

 reasonable doubt can be entertained that the variation in question is of a 

 comparatively trivial character, affecting the individual independently, and that it 

 is not stamped by correlation with other structural modification with sufficient 

 importance to justify the forms being separated from the species, or even a name 

 being given to the variety. All the examples referred to in the foregoing remarks 

 and figured in the plates accompanying this Memoir are from the Upper Chalk, 

 and were obtained from the same locality near Bromley. 



The fine specimen with large and greatly produced ultimate plates drawn on 

 PI. XI, fig. 2 a, is also an example of the second variation in the structure of this 

 species, to which I have alluded. This manifests itself in the presence of an 



