114 



FOSSIL ASTEROIDBA. 



TEXT-FIG. 5. Pedicellaria of Nymphaster 

 oligoplax. 



spine-pit, thus giving the plates an embossed appearance. This type of ornament 



I call the " pustulate " type. 



In previous portions of this Monograph both Mr. Sladen and myself have 



assumed that if no spine-pits are present on a plate they have been weathered 



away. It now appears that the absence 

 of spine-pits is such a constant character 

 in certain species that this supposition 

 can no longer be held, and the absence of 

 spine-pits indicates an original absence 

 of spines, or, at any rate, spines articu- 

 lated to the plates. In support of such a 

 conclusion it can be urged that, generally 

 speaking, Chalk fossils are but little 

 weathered, and that there is evidence 

 derived from a study of recent forms. 



Pedicellarix. As can readily be under- 

 stood, only pedicellarias which are articu- 

 lated in depressions of the plate are pre- 

 served in Cretaceous Asteroids. Purse- 



r 



like, valvate pedicellariaB (Text-fig. 5) 

 of a generalised type are common to 

 many genera. More specialised pedicel- 



lariaB, however, peculiar to the genera Metopastcr and Pycinaster are also met 



with (Text-fig. 7). 



TEXT-FIO. 6. Three pedicellaria from Pentago- 

 nasler quinqueloba on the right. A pedicellaria 

 from Hadranderaster abbreviatus on the left. 



"t 



TEXT-FIO. 7. A pedicellaria from Metopaster 

 Parkinsoni in the centre, on the left a pedi- 

 cellaria of Pycinaster senonensis, on the right a 

 pedicellaria of Staiiranderaster coronatus. 



KEY-TABLE FOR THE IDENTIFICATION OF CRETACEOUS 



ASTEROIDS. 



The following key-table is based on the shape of the marginal plates and the 

 character of the spine-pits on them. Generally speaking both superior and 

 inferior series are similar in these respects, but when otherwise a note is made in 

 the table. 



A short description is also given of various plates which cannot be adequately 

 treated in the table. 



All the plates figured in the table are magnified 4 diameters. 



It is convenient to consider the Chalk (Cenomanian-Senonian) species 

 separately from the Upper Greensand forms. No Cretaceous Asteroidea have 

 been described from below this horizon. 



