BY JAS. P. HILL. 207 



the point of articulation of the propterygium with the shoulder 

 girdle. The entire anterior portion of the left fin, supported by 

 the propterygium and its rays, is thus entirely free from the body. 

 On both sides, and especially on the left, the propter3^gia are 

 directed markedly outwards. 



The only parallel for this condition among living Elasmo- 

 branchs appears to be found in the Angel-fish {Rhina squatina). 

 In that form, as is well known, the anterior ends of the expanded 

 pectoral fins extend forward as two short horns supported by the 

 propterygia, and entirely free from the body wall. On the left 

 side of our specimen, except for the greater forward extension of 

 the fin, the condition in Bhina is essentially realised. 



In the abnormal Thornback described by Dr. Traquair the 

 anterior extremities of the pectoral tins projected as two short 

 processes, one on either side of the snout. I have found a similar 

 condition in one of a series of twelve young taken from a single 

 female Hypnos suhnigrum. In this specimen, w^hich measured 

 6*1 cm. in length, the anterior ends of the pectoral fins projected 

 as tw^o blunt horns, one on the outer side of the anterior portion 

 of each electric organ. 



Similar cases of the non-adherence of the anterior extremities 

 of the pectoral fins to the head have been recorded by Yarrell"*^ for 

 Raia clavata, by Dayl: for R. clavata and R. hatis, and by 

 Bureau; for R. asterias. All these cases are of the same nature, 

 and of all recorded instances of this abnormality that of the 

 Trygomwhina herein described is perhaps the most marked. The 

 meaning of this variation, to which some slight importance may 

 be attached from its occurrence in three distinct Batoid genera, 

 is not far to seek. Prof. Howes, in his paper§ on the fin-skeleton 



* Yarrell. British Fishes, ed. by Richardson, 1859, Vol. ii. p. 585 and 

 p. 384. 



t Day. British Fishes, Vol. ii. p. 345, PI. clxxi. fig. 2, and p. 337. 



X Bureau. Bull. Soc. Zool. France, 1889, xiv. p. 313, and fig. (References 

 from Bateson. Materials for the Study of Variation, p. 540.) 



§ Observations on the Pectoral Fin-skeleton of Batoid Fishes. P.Z.S. 

 1890, p. 680. 



