Holt and Caldeewood — Report on the Rarer Fishes. 388 



position. There is therefore a regular alteration of rows of sti'ong and feeble 

 spines. 



If now we observe the condition of these rows as the size of the fish increases, 

 we notice that gradually, the spines which were large become insignificant, and vice 

 versa. This change may, perhaps, be more readily understood by a comparison 

 of diagrams A and B, where in each case the large spines are marked o, and 

 the small spines +. 



By a continuation of the same process the arrangement of the spines still 

 alters. The small spines (+) of B become more and more reduced in size, and 

 the central row gradually disappears. The ray is now about 12 inches in trans- 

 verse measurement, according to our Irish specimens. The other small spines also 

 become reduced, and when a condition of sexual maturity is reached we find that, 

 in accordance with what has already been said as to the female being the more 

 spinous sex, the small spines of the male disappear rather more rapidly than those 

 of the female. In this manner a sexual difference becomes established in the 

 adults, which may be illustrated in the following diagram: — 



G C 



DiAG. 2. — Sections of tails of male ( $ ) and female (2) It- circularis in adult condition. 



The central row of spines is absent in both cases, and in the male all small 

 spines have disappeared. In the female the row of small spines is still indicated. 

 This row, however, does not extend in old specimens more than halfway along 

 the tail. 



Owing to the rate of growth being subject to considerable variation in different 

 localities, it may very readily happen that the size (12 inches) given above for the 

 disappearance of the central row of spines is too small, that in other localities, as 

 the North Sea, the rays may be larger before this row disappears. The size, as 

 stated, however, seems to represent a fair average in regard to Irish specimens. 

 At the same time there is, in the large collection of rays and skates under review, 

 a specimen of 12 inches — a female, which has still a distinct central row. The 

 specimen has no alar ocellus, or circular spot on the wings, and will be referred to 

 again. 



Another species which shows, to a marked extent, the variation which exists 

 amongst the linear spines, is R. microcellata. The female shows a condition 



