OPHIODERMA. 



141 



Dimensions. — Disk, one inch and three tenths in diameter; arms, four and a half 

 inches in length. 



Description. — A very imperfect figure of this Ophiura, under the name Asterias 

 spJimrulata, was first given by Young and Bird in their ' Geological Survey of the 

 Yorkshire Coast.' Professor John Philhps etched a good outline of it in his ' Geology 

 of Yorkshire/ and named it in that work, without, however, giving any description 

 thereof. Mr. Charlesworth, in the 'London Geological Journal,' 1847, pubhshed a 

 beautiful drawing of the fine specimen contained in the museum of the Yorkshire 

 Philosophical Society, and this fossil the council of that institution kindly communicated 

 to me for PI. XVI, fig. 4, of this work. Although this Ophiura is not uncommon in 

 collections of Yorkshire fossils, still it is rare to find good specimens in wdiich the details 

 of its structure are well preserved. 



The disk is flat, circular, or subpentagonal ; the radial plates, forming pairs, are placed 

 so far apart that the ten plates are nearly equidistant from each other (PI. XVI, fig. 4). 

 In this specimen the other parts of the disk are not preserved ; in another the body is 

 partly entire (fig. 3, a), and shows a series of small transverse plates extending between 

 each pair of radial plates towards the centre of the disk. The arms are long, smooth, and 

 nearly cylindrical ; they are three and a half times as long as the diameter of the disk, 

 and taper gently from the base to the apex ; their upper or dorsal surface is covered 

 with transverse plates, nearly four times as wide as they are deep (fig. 4) ; the ventral 

 plates are deeper in proportion to their width, as shown in specimen (fig. 2) j the lateral 

 plates are large and imbricated, as seen in a ray from Dr. Bowerbank's specimen 

 (fig. 3, h), magnified two and a half diameters. In this fossil the free outer margin of 

 the plates is slightly pectinated, and so fine are the spines that they are only visible 

 when magnified several diameters. In a specimen now before me, showing the base, 

 I find the buccal plates are each composed of four pieces ; one central element, Avhich 

 is the largest, is situated nearest the mouth, it has a square form rounded at the oral 

 edge ; two lateral pieces are placed on each side of the central element, and one triangular 

 plate at the outer part of the centrum, having its base applied to the central plate, 

 and its apex directed outwards towards the interbuccal spaces. The under surface 

 of the disk, seen between this triangular portion of the buccal plates and the border, 

 is clothed with small close-set imbricated scales. The buccal fissures are very narrow, 

 but their structure is not, for anatomical description, sufficiently exposed. 



Affinities and Differences. — In its general characters this species resembles Oplnoderma 

 Gaveyi, Wr. The disk, however, is proportionately smaller, and the rays are stronger 

 and rounder ; the dorsal and ventral plates of the arms are likewise longer transversely, 

 and shorter ; the lateral plates are less developed than the homologous parts in Opfao- 

 derma Gaveyi. 



