Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/558

 ginal plates and appendages thereof, the figure and arrangement of the discal plates, and the form and development of the rays : —

1. Astrogonium. Marginal plates large and smooth towards the centre, their inner border encircled by granules.

2. Goniodiscus. Marginal plates having the entire upper surface covered with close-set granulations.

3. Stellaster. Marginal plates granulated, the ventral segment supporting a pendent spine, the rays elongated and tapering to a lanceolate extremity.

Many of these characters are absent in fossil Goniasteridae, and are therefore valueless for palaeontological purposes ; for this reason I have retained the genus Goniaster for the large pentagonal short- rayed forms, and that of Stellaster for those with a smaller disk and more elongated rays.

This division must be considered merely provisional until we become better acquainted with the comparative anatomy of extinct forms. The very fine fossil discovered by my friend Samuel Sharp, Esq., F.G.S., in the Ironstone beds of the Inferior Oolite near Northampton, and which forms the subject of this communication, belongs to the group Stellaster, in consequence of the smallness of the disk, and the length and development of the rays. The absence of pen- dent spines or any indication of their presence, warns us, however, to be cautious in drawing hasty conclusions as to the true generic position of this Starfish, seeing that the presence of this spine is considered to be diagnostic of living Stellasters. Whether this fossil ever possessed such a spine or not, the mould does not enable me to make any positive statement anent.

Stellaster Sharpii, Wright.

Diagnosis. — Body pentagonal, sides arched, rays much elongated and tapering to a narrow extremity ; marginal plates thick, surface of the same finely granulated. Under surface of the disk covered with small close-set polygonal ossicles, having had apparently a very granular surface. The circumference of each ray surrounded by sixty pairs of marginal plates, which extend from the centre of the arch of one interradial space to the same point of the adjoining area. Ambulacral furrows wide, oral opening large.

Dimensions. — Diameter of the disk two inches from the centre of one areal arch to the same point on the opposite one ; from ray- point to ray-point six inches, depth of the border at the centre of the arch three tenths of an inch.

Description. — This remarkable fossil is entirely a mould in Iron- stone, none of the ossicles having been preserved ; but the sharp impressions of their forms and sculpture impressed on the Ironstone reveal a tolerably correct idea of the anatomy of the plates.

The Starfish rests upon its upper surface, which is firmly imbedded in the matrix, so that the size, shape, and character of the dorsal ossicles still remain to be discovered. Those on the underside of the disk are nearly uniform in size, and are small, pentagonal and hexagonal.

The granulations on the surface of these small bones appear to