92 



FOSSIL ASTEROIDEA. 



are generally similar in appearance. They differ, however, in measurements as 

 detailed below. 



Breadth of radialia .... 8 - 2 mm. 



Length „ „ 

 Breadth of supero-marginalia 

 Length „ „ 



Breadth of infero-marginalia 

 Length ,, ,, 



Breadth of ventro-lateralia 

 Length „ „ 



3-5 

 6-2 

 3-5 

 5-9 

 3-5 

 4-5 



It will thus be seen that the breadth of the plates diminishes as we proceed 

 ventralwards. The ridges, also, on the plates, become more rounded in the same 

 direction. 



The ornament of the plates consists of hemispherical granular prominences 

 of moderate size. They appear to have been especially prominent at the base 

 of the ridge. No spine-pits are present. 



The height of the ray is 16*5 mm., and the breadth is about the same. Post- 

 mortem contraction has brought the ventro-lateralia of opposite sides into close 

 approximation, in some cases totally obliterating the ambulacral groove. Along 

 one or two of the arms some of the adambulacrals are still visible. They are 

 1'8 mm. broad and P2 mm. long. The portion of the plate nearest the ambulacral 

 groove is depressed, giving the plate a two-storied appearance. 



A few robust, rhomboidal, smallish ventro-lateralia are present at the base of 

 the arm. Their breadth is 1*8 mm. They are mammiform. A few similar 

 plates also enter the base of the arm. 



The two collections of isolated ossicles presented by Mr. McPherson referred 

 to above are very interesting. Each specimen consists of a single ossicle 

 simulating one of the abactinal bulbiform ossicles of Pentaceros, but possessing 

 the distinct Arthraster ornament, associated with plates which exactly match the 

 ventro-lateralia of A. Dixoni and other plates which resemble the arm plates 

 of this species except that the surmounting ridge is not so high. There is no 

 doubt that the plates are those of a species of Arthraster. I have little hesitation, 

 in spite of their occurrence in the Upper Chalk, in referring the ossicles to A. 

 Dixoni, especially as it is a matter of common experience that species of Chalk 

 starfish have a wide stratigraphical range. 



Locality <m<l Stratigraphical Position. — Forbes' type is from the Lower Chalk, 

 Balcombe, Sussex. The specimen in the possession of Dr. Rowe was collected in 

 the zone of Terebratulina gracilis in Devon. The specimens presented to the 

 British Museum by Mr. McPherson are from the Marsupites zone, Brighton. 



