THE COPEPOD GENUS PARANTHESSIUS — ILLG 395 



known to science, I consider it is entirely probable that generic limits 

 will be subjected to review and revision on future occasions. The 

 group Modiolicola^ therefore, serves for the present to set aside ob- 

 viously related, well-characterized species which differ principally 

 from those of Paranthessius as follows : Absence of a rostrum ; posses- 

 sion of the highly characteristic antenna, with very much shortened 

 second segment, accompanied by elongation of the third and fourth 

 segments, and lack of armature on the third segment ; and the reduc- 

 tion of the armature of the maxilla to a single unornamented seta. 



There is a notable tendency to grouping of the species of Paran- 

 thessius within the limits set forth above. Subgeneric division is 

 hardly justified because of the existence of transitional species. In 

 six species the armature of the distal segment of the fourth endopodite, 

 consisting of some combination of setae or spines to the total number 

 of four or five, is developed. In the forms with four spines or setae, 

 further, the outline of the segment characteristic of the full armature 

 of five persists. In all these species there is reduction of the prehen- 

 sile function of the terminal antennal segment, the usual strong claw 

 being replaced by a group, typically three, of long setae, or weakly 

 clawed slim spines. In some of these examples, but not all, the prehen- 

 sile function is transferred to the penultimate antennal segment and 

 the terminal joint then reduced to palplike dimensions. 



The remaining species form a fairly coherent group characterized by 

 reduction of the distal segment of the fourth endopodite, both in gen- 

 eral outline, and in its armature, w^hich is reduced to two or three 

 spines or setae. In all but one of these species the terminal antennal 

 segment bears a heavy prehensile claw, and the penultimate segment 

 is the shortest and least developed. Paranthessius parvus (Norman 

 and Scott) bridges the distinction between these groups, however; in 

 that species, the prehensile function of the terminal segment is lost, 

 although the terminal endopodite segment of the fourth leg has the 

 spine complement reduced to two. 



In its body outline Paranthessius is characteristically cyclopoid, or 

 better, lichomolgid. An oval-shaped forebody is typical; in some 

 species there is a tendency to elongation, and in others toward in- 

 flation of the outline of this region to an approximate circle. 



The segment of the first swimming leg (the second thoracic seg- 

 ment — almost invariably called first segment by earlier authors) in 

 some species is joined to the cephalothorax, in others is a free segment 

 like the other pedigerous somites. The metasome then possesses three 

 or four free segments posterior to the cephalothorax. Thoracic seg- 

 ment 5 is much the smallest of all. Frequently the greatest width 

 of the metasome is at the first free secrnent. 



