1-2 EDWARD VII. SESSIONAL PAPER No. 22a A. 1902 



VI 



THE PAIRED FINS OF THE MACKEREL SHARK 



BY PROFESSOR E. E. PRINCE, Dominion Commissioner ok Fisheries, 



AND 



l)K. A. H. MacKAY, Superintendant of Education kor Nova Scotia. 



Preliminary Noti by the Director, Prof. Prince. 



In August, last year, a specimen of the Mackerel Shark {Lamna cornubica, Gmelin) 

 was brought to the Biological Station, then at St. Andrews, N.B. Dr. A. H. MacKay 

 was making a short stay at the Station and I suggested to him that the preparation and 

 study of the skeleton of the paired fins, especially the pectoral fins, would form a compact 

 subject which could be overtaken without involving labours too prolonged, and would 

 afford matter of some morphological interest. Dr. MacKay, with much skill, made two 

 most valuable preparations, and these with the drawings completed at the time, appeared 

 to me to furnish a basis for a short paper on the subject of the paired piscine limbs. 



With Dr. MacKay's consent I have combined his work and my own further studies 

 on his preparations and drawings, and it is necessary only to add that apart from the 

 general conclusions usually favoured by comparative anatomists to-day, the responsibility 

 rests upon me for the interpretation of the skeletal elements set forth in the following 

 brief report. 



GENERAL CONSIDERATIONS. 



The pectoral fins of Lamna cornubica are remarkable, even amongst the sharks, for 

 their great development and powerful muscular and skeletal characters. Instead of the 

 somewhat regular triangular form of fin as seen in Squalus [Acanthias), in Catulus 

 (Scyllium), in Scymnus, or even in A' otidanus, we find that while the fin is broad in 

 transverse width, it is greatly deepened in longitudinal extent, and presents a prolonged 

 lobate expanse, hanging far below the ventral contour of the trunk, and showing a cor- 

 respondingly strengthened, and expanded cartilaginous support. In its elongated 

 expanded character it recalls the pectoral limbs of the monstrous Selache maxima, or 

 Carcharinus lam,ia. Lamna, like its congeners, is a surface swimmer, and its breast 

 fins are in keeping with its pelagic mode of life. 



On examining the skeleton of the pectoral fins as figured in Plates V and VI we find 

 three regions defined, viz., a basal portion articulating, for the most part, with the 

 shoulder girdle ; a radial portion, made up of a series of jointed rods ; and a marginal 

 portion consisting of thickly massed horny fibres. The basal portion thus composed of 

 a small number of cartilaginous elements, forms the basipterygium, the morphological 

 nature of which has aroused much controversy. There is, however, a general agree- 

 ment as to its constitution. As the late Profes.sor Rollestonsaid,* ' the fore-limb consists 

 typically in Elasmobranchii of three basal cartilages, — pro-, meso-, and meta-ptery- 

 gium, articulating each with a facet on the shoulder-girdle : of one or two outer rows of 

 cartilaginous rods known as radialia, followed by horny fin-rays.' Ontogenetically these 

 basal elements and outer cartilaginous rods arise as a large flattened plate which breaks 

 up into the series of cartilages found in the fin of the adult fish. From the phylogenetic 



* Forms of Animal Life, 2nd Ed. Oxford 1888, p. 416. 



55 



