ASTROPECTEN. 



117 



to sixty dorsal marginal plates, which have a quadrate form, being rather broader than long; 

 they are Avell separated from each other, by reason of the convexity of their upper surface ; 

 the inner side of each ossicle is flat, the sides straight, and the outer margin and upper 

 surface rounded, which imparts a moniliform character to the border of the ray ; the convex 

 surface of the ossicles is crowded with minute granules, most conspicuous near the vertex 

 and disappearing on the sides. 



The ventral border plates (PI. X, fig. 1 a, h, c, d) have a rectilineal arrangement, and 

 gradually diminish in size from the angle to the apex ; on their posterioi" border is a row of 

 three or four little tubercles, which suppoi't short, stout spines (PI. X, fig. 1 c and d), 

 and many of these are preserved in situ. ; in the only specimen in which I have seen the 

 ventral surface exposed, these spines form a well-defined row, projecting outwards and 

 backwards from the distal border of the ventral plates (PI. X, fig. 1 h, c, d.) 



The upper surface of the disc in the specimen figured in PI. IX, fig. 1 a, exhibits 

 five prominent, oblong bodies, bilobed in structure, and having on their inner surface 

 a number of tooth-like processes, which mutually interlock. At fig. 1 ^ I have given an 

 enlarged view of these bodies ; they appear to be the upper portion of the large amlju- 

 lacral bones, for we observe a series of similar bodies occupying the middle of the rays in 

 other specimens where the matrix has been cleared away from these ossicles, as seen in 

 each of the five rays of the specimens figured in PI. IX, fig. 1 a, fig. 1 c ; and in another 

 specimen from the same locality (PI. X, fig. 3 d, fig. 3 d) the ambulacral bones are well 

 seen ; the serrated line between the two halves of the oblong ossicles is the suture at 

 the middle of the arch formed by the ambulacral bones ; the position and relation of this 

 median suture may be understood by referring to the section of a ray of Uraster ruhens 

 which I have given in fig. 32 a, page 99. 



The madreporiform tubercle is moderately large, and situated near the border 

 opposite one of the intermediate angles ; it consists of numerous fine, vertical laminas, 

 radiating from the centre, and which so closely resemble the septa of a Montlivaliia 

 that the tubercle might readily be mistaken for a small fossil coral attached to the 

 upper surface of the Star-fish. 



The ventral surface (PI. X, fig. 1 d) is shown in the only specimen I have seen with 

 the under side exposed; the mouth-opening (fig. 1 a), is surrounded by five petals, each 

 formed of two halves, and perforated in the middle for the passage of a tube, as shown in 

 the enlarged drawing (fig. I ^) ; at each intermediate angle there is an arch of small ossicles, 

 two and three rows deep, arranged in an imbricated manner, and over the summit of each 

 arch the point of one of the five petals rests ; this arrangement produces a curious 

 complicated structure, which is very well delineated in fig. .3 h ; the surface of all the little 

 ossicles entering into the formation of these arches is covered with small, spiniferous 

 tubercles. 



Within the ventral border plates are two rows of small, quadrate, spiniferous ossicles, 

 forming the outer walls of the narrow ambulacral furrows, the inner border of these 



