SILURIAN RADIATA. 175 



frequently dichotomizing at an acute angle^ branches very 

 rarely, but occasionally anastomosing ; cells extremely small 

 (eight in the space of 1 line), about eighteen rows in the width 

 of the frond, very close quincuncially arranged, without per- 

 ceptible linear disposition ; internal axis smooth, nearly mem- 

 branous. 



The extreme minuteness, number and close disposition of the 

 cells easily distinguish this species from the P. acuta or costulata, 

 and often give the specimens a resemblance to some fucoid. 



Not uncommon in the schists of Llansaintfraid ; schists of 

 Gelli Grin, Bala ; and schists of Corwen. 

 [Col. University of Cambridge.) 



Fenestella patula (M'Coy). 

 Sp. Char. Corallum forming small semicircular expansions about 

 half an inch in diameter ; interstices broad, strongly carinate, 

 slightly flexuous, and dichotomizing very frequently (usually 

 at rather less than every line in height), about seven in the 

 space of 2 lines ; fenestrules about one-third wider than the 

 interstices, of very irregular shape, but nearly equal in length ; 

 dissepiments strong, about half a line apart ; pores large, very 

 prominent (about three to the length of an interstice) . 



This species is more nearly allied to the Retepora {Fenestella) 

 antiqua of Goldfuss, than the Silurian species referred to it by 

 Lonsdale (F. subardiqua, D^Orb.) ; from the latter species it is 

 easily distinguished by its thick, widely divaricating and fre- 

 quently branched interstices, the irregular shape of the fenes- 

 trules, and the much fewer and more prominent pores. 



Not uncommon in the Wenlock limestone of Dudley. 



{Col. University of Cambridge.) 



Fenestella rigidula (M'Coy). 

 Sp. Char, Corallum formed of very slender ridged straight in- 

 terstices, branching seldom (at distances of about every 10 

 lines), several interstices often branching at the same height, 

 at a very acute angle, about ten interstices in the space of 2 

 lines ; mesial keel very strong, with a row of small pores ; 

 fenestrules very regular in size and shape, quadrangular, 

 slightly longer than wide, their width about equaling that of 

 the interstice ; usually three large thick -edged pores to the 

 length of a fenestrule and one larger at the end of each 

 dissepiment. 



In size, mode of branching, and rigid straightness of the in- 

 terstices, this is not unlike the F. subantiqua, but is completely 

 distinguished by its closer dissepiments, forming shorter fenes- 

 trules, and its much fewer and larger pores. From the row of 



