210 THE VOYAGE OF H.M.S. CHALLENGER. 



of Ophiochondrns (PL XLIII. fig. 13), whose arm hones are, however, utterly different in 

 other respects. Scarcely less curious are the mouth angles which are covered ahove by a 

 large, swollen, spongy peristomial plate, and this is continued down the outer open angle 

 by a film or veil of lime crust. Over the madreporic mouth shield this veil takes on the 

 form of a sort of rude pillar of spongy lime scales, or spicule. The closing of the open 

 angle of the mouth frame wings by a sort of crust is found elsewhere, but not in so 

 complete a form. The jaws and jaw plate are swollen and powerful, and the teeth thick 

 and cylindrical. The arm spines are solid with a peculiar wedge formation in cross 

 section ; and have a single row of thorns on each edge. 



This is a genus like an elaborated Ophiacantha and which, by its great radial shields 

 and its arm spines leans towards Ophiothrix, and yet one face of its arm bones has an 

 Astrophyton structure ! Here is one warning, among many, not to seek in Nature for a 

 regular, progressive and consecutive development. To suppose that this arm bone was a 

 last remnant of an Astrophyton progenitor, or the first hint of a future Astrophyton, would 

 be to drag the camel through the postern. (See PI. XLI. figs. 9-11.) 



Some of the species, at any rate, are viviparous. A new one, dredged in an expedition 

 of the U.S. Steamer " Blake," had a large bursa (PI. XL VI. fig. 4, Bit), in whose wall were 

 imbedded lime scales. Between this and the disk roof was a pocket (ovarial tube ?) 

 containing an embryo (8'), which was too macerated by alcohol to show much structure. 

 The wall of the bursa was joined with that of the digestive cavity (St). 



A section of the entire disk is given in Plate XL VII. fig. 5, showing the way in 

 which the ovarial bursas throw a fold over the digestive cavity. 



Ophiocamax vitrea, Lym. (PI. XIV. figs. 10-12). 



Ophiocamax vitrea, Lym., Bull. Mus. Comp. Zool., vol. v., part 7, p. 156, pi. viii. figs. 

 218-221, pi. ix. figs. 242-244, 1878 (young). 



Mouth papillae numerous, spine-like, arranged in a tuft. Tentacle scales blunt 

 spiniform, two or three to each pore. Disk covered above with rounded, equal, thin 

 scales set thickly with minute thorns and crochets. 



(Type specimen from Station 219.) Diameter of disk 17 mm. Width of arm with- 

 out spines 4 - 4 mm. About thirty, long, spiniform, equal papillae to each angle, set in 

 two or three ranks ; of which half a dozen are arranged under the teeth, and may be 

 considered tooth papillae ; and three on either side are borne on a small plate or scale, at 

 outer corner of mouth-slit, and cover the second mouth tentacle. Seven broad flat teeth, 

 with a rounded cutting edge. Mouth shields small, broader than long, with an obtuse 

 angle within and a truncated angle without, bearing a few short spines ; length to 

 breadth, l - 5 : 2. Side mouth shields exceptionally large and wide, meeting broadly 

 within, having their inner sides gently curved. Under arm plates with a wide, slightly 

 curved, somewhat swollen outer edge, and a narrow projection within, where they join 



