M. Robin on a peculiar Organ found in the Rays. 49 



discs according to the species, that in a transverse section of 

 the organ, from eleven to fifteen discs are counted in a R. clavata, 

 and twenty to twenty-five in the R. rubus and the R. batis. The 

 volume of the discs increases with age and the size of the indivi- 

 duals, but their number does not appear to increase. 



These gelatinous discs are piled up one upon another, in the 

 direction of the length of the apparatus, by the adherence of 

 their broad faces, with the interposition nevertheless of a thin 

 cellular partition. These longitudinal rows of discs are arranged 

 side by side, reunited by a cellular partition thicker than that 

 which separates each disc from that which follows or precedes it. 

 The kinds of longitudinal columns represented by the piled-up 

 discs are not rectilinear and do not all follow the length of one 

 of the faces of the apparatus ; but they are more or less contorted, 

 and are interrupted at intervals. The interruptions proceed from 

 the discs becoming at intervals irregular, more narrow, and the 

 series of discs terminates ordinarily in a very small, triangular 

 one. It follows from these anatomical arrangements, that on the 

 surface of the organ we may perceive one of the small faces of 

 each of the superficial discs, and study very regularly the elon- 

 gated, quadrilateral or lozenge-shaped polygonal, sometimes hex- 

 agonal form which it assumes in consequence of the reciprocal 

 pressure which it undergoes from the adjoining discs. We may 

 moreover very easily perceive that when the discs of a series 

 begin to lose their form and are interrupted, there exist at the 

 side other irregular discs which commence a new series. It is to 

 be remarked also that the discs are ranged more regularly on the 

 internal surface of the apparatus in the Rata rubus and R. batis 

 than in the Raia clavata ; in the first two species wc also observe 

 on the internal surface of the organ, that one of the partitions 

 which separates the series of discs on the internal surface, follows 

 its whole length and is of greater thickness than the rest : it is 

 formed by glistening aponem-otic fibres, and it forms a sort of 

 longitudinal pile into which the vessels and the nerves penetrate. 



With res))ect to the gelatinous substance of the discs, mag- 

 nified 400 diameters, it is seen to be hollowed out by cavities, 

 and the walls of the latter are hollowed by cavities gradually 

 lessening in size. The substance too which circumscribes these 

 areola; (to which wc shall recur hereafter) is hyaline, homogeneous 

 and transparent; it is studded with extremely fine molecular 

 granules. From one spot to another are very regular granular 

 spheres of 0"''"-0050, surrounded by a very pale circular mass of 

 granules similar to tlie preceding. It is impossible to recognise 

 veritable cellules with walls and nuclei, and it is easy to see that 

 the preceding areola; are not cellules ; we shall soon sjjcak of 

 their uses. On the margins of the discs, the homogeneous gela- 



Phil. Mag, S. 3. Vol. 30. No. 198. Ja7U 1847. E 



