THE 



GEOLOGICAL MAGAZINE, 



NEW SERIES. DECADE III. VOL. IV. 



No. Ill— MARCH, 1887. 



0:RX(3rXl<TJ^JL, J^I^TIOXiBS. 



I, -On a New Ophiurella prom the Calciferous Grit, near 

 Sandsfoot Castle, Weymouth, Dorset.^ 



By the late Dr. Thomas Wright, F.E.S., F.G.S. 



(PLATE III.) 



Genus Ophiurella, Agassiz, 1836. 



DISK small, membranous, often indistinct, a character which 

 separates this genus from Ophiiira. Eays very long, slender, 

 depressed, formed of circles of plates, four in each circle ; the 

 lateral plates are the largest, most prominent, and provided with 

 long spines ; the basal plates are small and spiniferous, and the 

 dorsal smooth and without clothing. Mouth plates small and 

 triangular. All the species known have been found in the Jurassic 

 rocks. 



Ophiurella nereidea, Wright, 1880. (PI. III.) 



Description. — Disk small, irregularly penta-lobed, each lobe con- 

 sisting of a shield-like elevation formed by the radial plates, which 

 are covered by a tegumentaiy membrane closely studded over with 

 small granules ; the interlobular integument is entirely absent, 

 having apparently, if it ever existed, been destroyed in the process 

 of fossilization. 



The arms, or rays (five in number), are long, four times the length 

 of the disk's diameter. They do not taper much between the 

 radial plates and their termination, and consist of innumerable 

 highly movable I'ings composed of: — 1st a centro-dorsal plate, 

 which, with its fellows, form a long, smooth, convex, continuous 

 chain, flattened at the summit, and laid along the middle of the rays ; 

 2nd, two rows of lateral plates which bend downwards, closel}^ clasp- 

 ing the sides of the ray ; each plate supporting a small tubercle, on 

 which a stout thorn-like spine is articulated by a kind of ball and 

 socket joint ; 3rd, the basal or ventral plates which close the ray 

 below are small and much concealed, they likewise carry short, 

 stout spines. One of the spiniferous arms of the Ophiurella, as 

 it lies on the slab of calcareous grit before me, resembles a mai'ine 

 worm, the Nereis nuntia, and hence tlie origin of the specific name 

 I have ventured to give to this Brittle-star. The arms are very 

 much bent and curled, so that this species may be said to have had 

 highly moveable arms. 



1 Extracted from the Dorset Natural History and Antiquarian Field Club, vol. ir. 

 p. 56, printed at the "Journal" Offices, South Street, Sherborne. 



DECADE III. VOL. IV. KO. III. 7 



