146 LIASSIC OPHIURID^. 



Ophioderma tenuibrachiata, Forbes. PI. XVIII, fig. 5, a, b, c. 



Ophioderma tenuibbachiata, Forbes. Proc. Geol. Soc, vol. iv, p. 233, fig. 5. Read 



Nov., 1843. 



— — Charlesworth. Lond. Geol. Journ., pi. xix, fig. 1, 1847. 



— — Morris. Catalogue of British Fossils, p. 84, 1854. 



— — Pictet. Paleontologie, torn, iv, p. 274, 1857. 



— — Wright. Brit. Association Reportforl856, p. 403, 1857. 



— — Dujardin et Hup6. Hist. Nat. des fichinod., p. 234, 



1862. 



Disk small, flat, subpentagonal ; arms long, delicate, and tapering little. 



Dimensions. — Disk, four tenths of an inch in diameter ; arms, two inches and seven 

 tenths in length. 



Description. — The body of this species is smaller than that of Opldoderma Egertoni, 

 and the rays in proportion are longer and less tapering. " They have a more flexible 

 aspect than those of 0. Egertoni, and present in their section a different form of the 

 central ossicula; for these, instead of being trilobate, are oblong, mth a triangular 

 central anterior lobe."^ In the fine specimen I have figured the ventral surface of the 

 rays has the marginal angle much more acute than the homologous part exhibits in 

 0. Egertoni. 



Affiyiities and Differences. — This species may be readily mistaken for 0. Egertoni ; 

 when carefully examined, however, it is found to possess a smaller disk, longer and less 

 tapering rays, having a more acute angle at the margins of their ventral surface ; a 

 section of the central ossicula exhibiting a bilobed outline, whereas in 0. Egertoni a 

 similar section shows a trilobed form. 



Locality and StratigrapJiical Position. — This species, like the preceding, is found on 

 the surface of the large blocks of hard micaceous sandstone described by me as the Star- 

 fish-bed of the Middle Lias at Down Cliff, near Bridport Harbour, Dorsetshire. (See p. 

 145.) It is associated with Ammonites fimbriatus. Sow., Amm. margaritatm, Mont., Belem- 

 nites elongatus, Mill., and Opldoderma Egertoni, Brod. 



History. — First discovered by my old friend the late Dr. Murray, of Scarborough, and 

 communicated by Dr. Bowerbank, E.R.S., to the late Professor Forbes, who first described 

 it in the 'Proceedings of the Geojpgical Society' for 1843. The very fine specimen 

 figured in PI. XVIII belongs to my cabinet. 



1 Forbes, 'Proc. Geol. Soc,' vol, iv, p. 233. 



