ARTITRASTER CRI8TATUS. 93 



Remarlts. — Forbes coniparetl the genus Arthrastvr with the iiiodorn genus 

 Ophiilmster. The hu'ger amount of material known since that time does not allow 

 us to recognise such aniuity. 



Valette (see above, p. 81) has described certain isolated plates, which are 

 similar in form and size, as belonging to the genus A rtJi raster, and has called 

 the species .1. spiinnniKti^. These jilates are smooth and show no trace of the 

 surmounting longitudinal ridge or oi'nament which is so characteristic of Arilirastcr. 

 Dom Aurelien Valette kindly lent these plates to Dr. Bather in order that I might 

 examine them. T am therefore enabled to state that the plates arc those of 

 Pyciuaster augustatus. 



2. Artukastki; riasr.vTLs, n. sp. I'l. XXIX, figs. 10, 10(^ 10 b. 



Specific Characters. — Ridges of the radialia and supero-marginalia cristate. 

 Upper surface of the ridge of all arm-ossicles possessing lipped pits formerly 

 occupied by small spines. 



Matenal. — The specimen figured on PI. XXIX was restored l)y Dr. Blackmore, 

 of Salisbury, from a nuiiil)er of isolated ossicles in his collection which were found 

 in a single mass of chalk. These ossicles are tlic only material known of the species. 



Description. — Tlie dimensions of the ossicles are as follow : 



Breadth of radialia . . . o'l mm. 



Length „ „ . . . 3-2 „ 



Breadth of supero-marginalia . . -i'8 ,, 



Length ,, ,, . . 3-2 „ 



Breadth of infero-marginalia . •4'2 ,, 



Length „ ,, • • 3-2 „ 



Breadth of ventro-lateralia . . 3"8 „ 



Length 



o 



o 



Just as in A. Divoiii the breadth of the plates diminishes ventralwards, and the 

 ridges on the plates become more rounded in the same direction. The cristate 

 ridges of the more dorsal plates are, however, very characteristic of the species, as 

 are also the lipped pits on the summit of the ridges. The pits were formerly 

 occupied by small spines. The base of the ridges of the plates possesses the 

 granular hemispherical prominences, such as are also met with in A. Dixoni and 

 characterise the erenus. 



o* 



JmciiIUij and ,Strati(jraphical I'onUiun. — Micheldevei-, Hunts. Zone of MicraKter 

 cur-anguiaum. 



