M. Robin on a peculiar Organ found in the Rays. 19 



Ohs. 2. R. Giintcri is referred by Arrhenius to R. glandulosus, 

 to which it is doubtless very nearly allied. Its armature seems 

 to differ and so does its panicle. Probably R. thyrsiflorus (Weihe) 

 is only a form of this species ; and that, together, they will take a 

 place close adjoining to, but not absorbed in, R. glandulosus. 



N.B. This plant is named, on my recommendation, R. glan- 

 dulosus var. subracemosus in the ' Fasciculus of Rubi,^ issued 

 lately by the Rev. A. Bloxam. 



III. — On a peculiar Organ found in the Rays (Raia, Cuv.). 

 By M. LE Dr. Ch. Robin. 



There exists upon each side of the tail of the Rays an organ 

 which is not mentioned in any of the works which I have hitherto 

 been able to consult. This apparatus however deserves, on more 

 than one account, to attract the attention of physiologists, and 

 probably of physicists also. 



The two organs united form nearly the third of the entire bulk 

 of the tail of the Rays. The bulk of each, at its largest part, is in 

 one of these fishes of an average size, nearly that of the index-finger. 

 Their origin is towards the union of the first and second quarter 

 of the caudal appendage of the Rays, and they terminate in a point 

 at the extremity of the latter. Their anterior extremity is soft 

 and more or less slender, according to difi'erent individuals : it 

 swells gradually as far as the middle of the tail of these fishes ; 

 the volume remains the same as far as the origin of the posterior 

 quarter, whence it diminishes finally to the end. This organ is 

 at first almost cylindrical, though a little flattened on the sides 

 (to about the extent of the anterior quarter) ; in the whole of 

 that part it is enveloped by several thin and concentric muscular 

 layers. These muscles soon terminate in as many aponeurotic 

 layers ; the organ then becomes subcutaneous, and at the same 

 time its form changes, becoming round externally and flattened 

 within. In a word, this organ, separated from the tissues to which 

 it adheres, presents the form of an elongated fusiform body, 

 swollen in the centre, more or less blunt at the extremity, and 

 always flattened upon its internal surface. 



In a Ray one metre* long, the tail was 49, and the organ 36 

 centimetres in length ; 11 millimetres in the transversal direction, 

 and 13 from above downwards. 



The tissue of this organ has the semitransparency of gelatine, 

 but more consistence, and its colour is a clear translucid gray. 

 It is furnished with a general fibrous envelope, which adheres to 

 the adjoining tissues by aponeurotic membranous layers ranged 

 at regular distances. 



* The metre is equivalent to 3937 EngUsh inches. 



2* 



