SPELEROPHTHALMUS. 73 



Genus SPHiEROPHTHALMUS, Angelin. 



The genera Leptoplastus, Sphxrophthalmus, Ctenopyge and Eur year e form a 

 natural group of the Olenidse, characterised by the fact that the cheek-spines do 

 .not spring from the posterior angles of the head, but from the middle of the 

 external margin or even from the anterior lateral angles. In many other forms, 

 even in so typical an Olenus as 0. truncatus, the posterior margin bends towards 

 the front near its outer extremity, so that the genal angles are placed rather 

 forward, but the character is greatly exaggerated in the genera under consideration. 

 In some of the species there is still a definite genal angle although it lies far 

 forward ; in others there is no angle at all and the cheek-spine starts abruptly from 

 a perfectly rounded margin. 



Brogger considers that all these forms should be placed in a single genus, 



which he calls Leptoplastus and which he divides into four subgenera as follows : l 



Head moderately wide, Head generally very 



cheek-spines short. wide, cheek-spines very 



a ,. T long. 



Section 1. a 



Pleuras ending in short 



points. Tail minute, gene- Leptoplastus (sens, strict.). Eurycare. 



rally armed with short 



spines, wide. 



Section II. 



Pleuras in posterior 

 part of the body geniculate 



towards their extremities, Sphaerophthalmus. Ctenopyge. 



produced into long re- 

 flexed spines. Tail small, 

 entire (or ?), longer. 



In formulating this scheme Brogger adopts a somewhat doubtful interpretation 

 of the tail of Ctenopyge pecten, which will be discussed later. According to the 

 usual view the tail of that species is neither small nor entire, and its breadth is 

 considerably greater than its length. 



Persson's grouping of the same genera has already been given on p. 51. The 

 chief difference in his definitions is that he leaves the tail out of consideration, and 

 that he differentiates Sphswophthalmus' from Ctenopyge by the greater convexity of 

 the glabella and cheeks instead of by the width of the head. 



Unfortunately at present no grouping of the species into genera can be more 



1 Die Silur. Etagen 2 und 3, p. 113. Brogger's table is in Latin, which is here freely translated. 



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