116 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



covered with granules. Should it be shown hereafter that this interpretation is 

 incorrect, and the radial shields are free from granules, theu P. vestita belongs 

 in the genus here called Ophiarachuella, which then becomes Pectinura, whUe 

 Pectiuura, as here used, would then take the name Ophiopeza Peters, since the 

 type O.fallax is a typical member of this group. 



This is a natural and, except for P. danbt/i, a very homogeneous group of 

 shallow-water forms, confined to the eastern Mediterranean, Indian Ocean, East 

 Indies, Australia, and New Zealand. Tliey are to be distinguished from each 

 other chiefly by the number, length, and appearance of the arm-spines, but the 

 granulation of the disc and the form of the upper and under arm-plates may fur- 

 nish excellent characters. 



Key to the Species of Pectinura. 



Arm-spines blunt, or slightly acute, middle ones not longest. 

 Arm-spines few (4-8). 

 Arm-spines 6-8. 

 Upper arm-plates " somewhat orbicular ; " under arm-plates 



fan-shaped ; arm-spines about as long as joint vestita 



Upper arm-plates at first squarish, becoming fan-shaped on outer 

 part of arm ; under arm-plates not fan-shaped ; arm-spines 



about I joint •. . . • cyJindrica 



Arm-spines fewer. 



Arm-spines 5, about | joint exilis 



Arm-spines 4, nearly twice as long as joint danbyi 



Arm-spines numerous (9-14). 



Disc granulations coarse (35-200 per sq. mm.) ; arm-spines | joint 

 or longer. 

 Upper arm-plates not twice as wide as long; supplementary 

 oral plates well marked ; arm-spines about \ joint, lowest 



distinctly longest arenosa 



Upper arm-plates three times as wide as long or wider; sup- 

 plementary oral plates often wanting. 

 Size large, disc diameter up to 40 mm. ; arms conspicu- 

 ously spotted with purple ; oral shields usually longer 

 than wide ; lowest arm-spines largest but not necessarily 



longest maculata 



Size smaller, disc diameter seldom up to 2-5 mm. ; arms 

 not at all spotted or marked with purple. 

 Arm-spines subequal ; oral shields rather wider than 



long assimilis 



Lowest arm-spine decidedly longest; oral shields 



about as long as wide aerjualis 



Disc granulations very fine (350-500 per sq. mm.) ; arm-spines 



scarcely I joint fallax 



Arm-spines sharp, middle ones distinctly longest yoldii 



