8 OPHIOMUSIUM TESTUDO. 



many of them rudely hexagonal, and each bearing usually from one to four 

 large grains. Almost the entire margin of each interbrachial space is occu- 

 pied by a thick swollen plate having a depression in its border; it descends 

 to the lower surface, where it joins the mouth-shield ; the covering of the 

 lower interbrachial space is completed by two massive genital scales which 

 run from the mouth-shield to join the outer corners of the radial shields, 

 which are so large and swollen as to resemble the halves of an egg. They 

 touch at their middle point, but diverge both outward and inward. Within 

 they are separated by a large disk-plate, and without by a triangular button 

 which rests on the top of the arm ; length to breadth, 1.8 : 1.4. Arm-spines 

 two, cylindrical, tapering, about .4 mm - long. Besides these there is on the 

 upper surface of the side arm-plate a supplementary spine somewhat longer 

 than the others. Tentacle-scales are only on the second and third plates ; 

 they are circular, and situated at the inner corners. The tentacles are 

 slender, pointed, and smooth (Fig. 6 |. 



Color, in alcohol, white. 



Hassler Expedition; Barbadoes, 100 fathoms. Dr. William Stimpson ; 

 West Florida. 42 fathoms. 



This species agrees with 0. vaUdum Ljn. as to its under arm-plates, but 

 differs in having supplementary arm-spines, only two regular arm-spines, 

 and grains on the disk. 



OpMomusium represents the maximum development of external plates, and 

 the minimum of internal skeleton. The figure shows how poorly formed are 

 the mouth-frames, jaws, and jaw-plate; and t lie arm-bones are still more 

 insignificant. Instead of the strong disks found in genera witli delicate 

 skins, such as OpMomyxa and Ophiura, there are only weak bones, which, in 

 profile, are pinched in the middle like an hour-glass. 



Ophiomusium tostudo »p. dot. 



/'/. /. /. i 



Marks. — Two or three minute arm-spinea No under arm-plates 

 beyond the third. 



/' cripHon of an Individual. — Diameter of disk, '>.'<""". Width of arm, with- 

 out Bpines, I B^"-, close to disk. Length of arm, l'J'r l . Mouth-papillae four 

 "ii each side, and one diamond-shaped, at the apex of the jaws, all forming a 

 closely soldered line. Mouth-shields longer than broad, pentangular, with a 



