PEXTACEROS. 89 



Specific Characters. — Primary radialia (or interradialia) large with a dilated 

 summit which possesses no ornament and is excavated into pits. " Ossicles of the 

 arm are narrow, slmttle-shaped, tumid m tlie centre and slightly impressed 

 towards each extremity " (Forljes). 



Material. — Several fragmentary remains of this species are known. The most 

 nearly perfect remains are those in the Museum of Practical Geology, Jermyn 

 Street. Other specimens are in the British Museum (Natural History), registered 

 E. 5037, 57624, E. 2564 (all PI. XXV, fig. 5), 7600, E. 25637, E. 2565. 



Description. — X^othing is known further than the description given in the 

 diagnosis. Forbes' description reads as if he had described the species from the 

 specimen in the Museum of Practical Geology. This originally was in the collec- 

 tion of the Marquis of Xorthampton. Forbes seems to have described the large 

 ossicles upside down. He also says they were in a circlet of five, which is not 

 apparent in any specimen known. The roughened and pitted surface recalls in 

 some respects the jn-iniarv I'adialia and interradialia of P. coronatus. 



Ldcalitij and Striitiijrdjiliiral I'usiticn. — Upper Chalk, Kent and Sussex. 



in. Pextaceros, sp. PI. XXV, fig. 7. 



This specimen is preserved in the British Museum of Xatural History (no. 5514). 



It consists of five marginal plates which are 12"3 mm. high, and have an 

 average length of 5*5 mm. The plates are rugged in appearance and the orna- 

 ment is worn away. A cirral of a crinoid (probably Boiirgttetiocrinus) has become 

 fixed between two of these plates. 



1 1. Pentacekos, sp. PI. XXV, fig. 8. 



The only specimen is presei-ved in the Brighton Museum. It consists of a few 

 marginal plates. The supero-marginals are rather irregular in shape, some being 

 almost wedge-shaped. On an average they are I iniii. high and 3'2 mm. long. 

 The infero-marginals are opposite and equal in length to the supero-mai-ginals. 

 They are only 2-it iniii. high. The plates possess a distinct margin, but the 



