86 FOSSIL ASTEROIDEA. 



madreporiform striations on the truncated summits and the ocellato-punctate sides 

 give a most characteristic appearance and render the species unmistakably distinct 

 from all known species of Pentaceros. 



The ambulacral groove is 3*5 mm. wide. The adambulacrals are difficult of 

 recognition and have probably for the most part been lost, but a large number of 

 the hour-glass shaped ambulacrals may be seen. 



Locality and Stratigraphical Horizon. — Upper Chalk, Kent; Upper Senonian, 

 Brighton. 



6. Pentaceros abbreviatus, n. sp. PL XXIV, figs. 1, 1 a, 1 b, 1 c. 



Specific Characters. — Body of medium size. Arms moderately produced, but 

 their breadth making them appear stumpy, rounded at the extremities, and hemi- 

 spherical in cross section. Five series of dorsal ossicles enter their base. Of these 

 the radialia and adradialia persist throughout the length of the arm. A few small 

 intermarginalia are present. 



Material. — There is only one specimen known of this species, and of this 

 practically all that remains are two arms. It is preserved in the British Museum 

 of Natural History (J. Tennant's Coll., 57538). 



Description. — These arms are characteristically wide, the width of the arm at 

 the base being 31 mm. They narrow very gradually towards the extremity. 

 Throughout the ray the ossicles, except for the differences noted below, are very 

 similar in appearance. At the base of the ray, where dorso-lateralia also enter into 

 the composition of the dorsal skeleton, they are rounded and possess interspaces 

 of considerable extent. These interspaces are often filled by smaller granules 

 arranged irregularly. At times, however, between two radialia or adradialia one 

 of the smaller ossicles is arranged in a very regular and alternating manner. Both 

 large and small ossicles are finely granulated, and the large ossicles alone are 

 perforated for pedicellarise. The average size of the larger ossicles at the base of 

 the arms is about 6 mm. Towards the extremity of the ray the radialia, adradialia, 

 and marginalia become hexagonal, and fit very closely so as to make a compact 

 skeleton. The terminal ocular plate is hexagonal and conspicuous. It has a 

 flattened articulation which undoubtedly was originally occupied by a spine. 

 Several of the other dorsal plates in the distal portion of the ray also possess 

 similar articulation. 



