154 



OOLITIC OPHIURIDiE. 



Genus — Ophiurella, Agassiz. 



Disk small, membranous, and often very indistinct ; rays long and slender ; lateral 

 ray-plates provided with elongated filiform spines. All the species hitherto discovered 

 belong to the Jurassic series. 



Ophiurella Griesbachii, Wright. PL XVIII, fig. 3, a, h. 



Ophioderma Griesbachii, Wright. Ann. and Mag. Nat. Hist., 2nd series, vol. xiii, 



pi. xiii, fig. 2, 1854. 



— — Forbes, in Morris's Catalogue of Brit, Fossils, 2nd edition, 



additional species, 1854. 



— — Wright. Brit. Association Reportfor 1856, p. 403, 1857. 



Disk small, membranous, irregularly pentagonal ; rays long, round, slender, and 

 gently tapering ; ventral ray-plates moderately large and pentagonal, lateral large, in the 

 form of oblique shields clasping the sides of the rays in an imbricated manner, and 

 supporting short stout spines ; buccal opening star-shaped, surrounded by a series of 

 blunt osselets. 



Dimensions. — Diameter of the disk, seven twentieths of an inch ; length of the rays 

 from peristome to apex, three quarters of an inch. 



Description. — This beautiful little Brittle-star of the Oolitic sea was discovered by 

 my friend the late Rev. A. W. Griesbach, of WoUaston, to whose kindness and liberality 

 I am indebted for the series of specimens I possess, and by which I have been enabled 

 to make out the structure of this fossil. The disk is small and often very indistinct, 

 consisting of five pairs of heart-shaped plates, so closely united together that in some 

 specimens it appears to be formed of a single circular disk. Each pair of plates has a 

 heart-shaped form, and the small corresponding ray stands out in bold relief from the 

 under side of the disk. All the specimens I have seen lie on their upper surface, with 

 the ventral exposed, so that the clothing of the disk is concealed ; in one specimen, 

 however, where a portion of one of the plates is weathered, I observed with an inch-object- 

 glass under my microscope the impression of a series of small imbricated scales resting 

 on the matrix. The rays are long, round, slender, and gently tapering, about three times 

 the length of the diameter of the disk ; their under surface, the only one exposed, exhibits 

 in weathered specimens a central element of an elongated form, resembling in miniature 

 the elongated centrum of the vertebra of a fish. PI. XVIII, fig. 3, h shows this structure 

 magnified three and a half times. In the only unweathered specimen I possess, from the 



