Holt and Calderwood — Report on the Rarer Fishes. 381 



with a view of drawing attention to certain less conspicuous, but equally 

 important differences occurring not only between males and females but between 

 old and young specimens of the same species, the following observations have 

 been introduced : — 



(a) The Relative Convexity of the Anterior Margi^i of the Disk. — This feature 

 is generally referred to as of specific value, without much reference as to 

 the maturity or sex of the examples described. Day,* for instance, in dealing 

 with R. batis, says : — " The anterior edge of the disk undulated and deeply 

 emarginate below a line drawn from the end of the snout to the angle of the 

 pectoral fins." A similar statement is made with regard to R. macrorhynchus, 

 R. alba, R. oeyhrynchus, R. fullonica, &c., and in none of these cases are we told the 

 size or sex of the species to which the statements are applicable. In the case of 

 R. alba we are indeed told that " In young specimens the disk is not so undulating 

 along its border as in adults ;" but again, we are left in ignorance as to the sex of 

 the young specimens. Moreover, his description of the adult R. alba has in all 

 probability been drawn up from a male example, which, following the course to be 

 observed in all Ratidae, has a more deeply concave margin than the adult 

 female. Even yet we may not conclude that his young specimen was a female, 

 because young specimens of both sexes show less concavity in their anterior 

 margins than adults. 



(b) The arrangement of the Dorsal and Caudal Spines. — The dorsal and caudal 

 spines, or dermoid denticles, are invariably arranged in rows, and may be spoken 

 of as "linear spines." They form, in their arrangement, a conspicuous featm-e in 

 the appearance of different species of the genus Raia. 



Spines are also found conspicuously on the snout, and round the orbits. In 

 certain species {R. clavata, R radiata) spines are scattered over the dorsum of the 

 disk, but in the majority of cases the denticles found in this situation are 

 insignificant, and they are never arranged in anj^ definite order. We find it 

 convenient to term these lesser denticles " spinules." They are stronger and 

 more conspicuous in the female than in the male, except in the case of R. radiata, 

 where the spines of large size are freely distributed over the dorsum, and where 

 it may be said that no difference, in spinulation, exists between the sexes. 



Spinules are present on the tail also, scattered between the linear spines, or 

 formed into a sub-marginal border. The latter condition is seen in R. circularis, 

 adult R. alba, R. radiata, and R. microcellata. In the description which follows as 

 to the caudal spines these spinules are not considered. 



One other point must be noticed before treating the arrangement of the spines. 

 Apart altogether from the alar spines already referred to as prominent only in 

 mature male, it is interesting to notice that as sexual maturity is reached, i.e. as 



* Day, "British Pishes," vol. ii., p. 336. 



