152 



LIASSIC OPHIURID^. 



length of the arms ; its upper surface is covered with a series of small imbricated scales, 

 which are disposed in consecutive order from the margin to the centre ; a double series 

 of small scales are seen on the interbrachial space of the under surface (PI. XVII, fig. 2) ; 

 the arms are short, broad, and tapering, not quite twice and a half the diameter of 

 the disk ; the dorsal ray-plates are narrow and heart-shaped, from the manner the lateral 

 plates clasp the rays (PI. XIV, fig. 1, Jj) ; the lateral ray-plates are proportionately large, 

 and encroach much upon the ventral series ; they have an inflated appearance and appear 

 to have supported numerous small spines, of which some obscure traces only remain ; the 

 ventral ray-plates are very small and triangular (PI. XVII, fig. 2, h, fig. 3, and fig. 4) ; 

 they resemble a series of heart-shaped pieces inserted between the lateral plates, and 

 united with them along the median line. The mouth-opening forms a star with five 

 branches, presenting well-marked buccal fissures (fig. 2, h) ; in one of the specimens it 

 is surrounded with small pieces disposed in a series. In the enlarged figure of this species, 

 given in PI. XIX, fig. 3, the disposition of the dorsal plates is much better shown. 



Jffmities and Differences. — The structure of the rays, in this Brittle-star, resembles 

 OpJnolepis Ramsayii, Wr., from the Lower Lias ; it has similar large lateral and small 

 ventral plates ; the absence of the disk, however, in 0. Bamsayii prevents a more perfect 

 diagnosis of that species, and a more accurate comparison with 0. Murravii, being made. 



Localify and Stratiyrajjhical Position. — This very rare little Brittle-star was discovered 

 by my friend the late Dr. Murray, of Scarborough, to whose liberality I am indebted 

 for the specimens figured in PI. XIV, fig. 1, and PI. XVII, fig. 2, both collected from 

 the Marlstone near Staithes, on the Yorkshire coast. The one figured in PI. XIX was 

 obtained from the Grey Limestone near Scarborough, and kindly communicated by my 

 friend J. Leckenby, Esq., P.G.S., to whose cabinet it belongs. 



History. — First figured and described by the late Professor E. Eorbes in the fourth 

 volume of the ' Proceedings of the Geological Society/ afterwards by Mr. Charlesworth in 

 the ' London Geological Journal.' 



Genus — Acroura, Agassiz, 1834, 



This genus, of which the organic characters are very imperfectly defined, was esta- 

 blished by M. Agassiz in his Prodrome for Ophiura prisca, Miinst. (PI. II, fig. 5), a 

 solitary species from the Muschelkalk. The form approaches much that of Ophiura 

 texturata, Lamk., but differs in this, that the rays are long and very delicate ; the 

 osselets of the arms are twice as long as they are broad, having their lateral borders 



