of the Fishery Board for Scotland. 



99 



and higher than in the male. In regard to the last feature, the 

 crenate edge of the carapace of the male is broader than that of the 

 female, and is also turned upwards. (Compare Fig. 10 (a, male ; b, 

 female), Plate 1., which represents the shells of two crabs of the 

 same size, viz., 6| inches.) Of the three sexual characters, the first, 

 viz., the external genital organs, is alone diagnostic for all crabs. 

 Thus, it is possible to distinguish the sex of a crab when it is 

 little more than a quarter of an inch in breadth. The second and 

 third characters are only applicable within limits. Thus, the 

 difference in shape of the abdomen will be found sufficiently defined 

 in crabs of 2\ inches and upwards; while the shape of the shell as a 

 sexual distinction is of little value except in crabs over 4 inches in 

 breadth. In other words, the last character is of moment only in 

 mature crabs. An additional feature which is especially noticed ^ in 

 crabs over 5 inches in breadth is the comparative size of the biting 

 claws in the male and female. In the former the claws are very much 

 larger than in the latter. Buckland mentions that in certain places the 

 fishermen were accustomed to send the claws alone of the large male to 

 market, the body being either used for bait or thrown away. While, 

 then, the external organs of generation are a primary sexual character, 

 the shape of the abdomen and shell, and the comparative size of the 

 large claws, are secondary diagnostic sex features. 



The testes and ovaries are paired ; there is, however, a communication 

 between the right and left testes, and also between the right and left 

 ovaries. 



Proportional Numbers of the Sexes. 



In order to arrive at the proportional numbers of the sexes, two 

 large groups of crabs have been examined — (1) The small crabs 

 which are found on the beach, and (2) the crabs caught in the creels of 

 the fishermen. The beach crabs were collected twice a month during a 

 year ; their sex and size were noted (Tables IV. and VIII.). The number 

 of male and female crabs in 20 catches of the fishermen at Dunbar were 

 counted. In the list each month of the year is represented (Table V.). 

 Statistics of the catches of crabs at Crail were kept by Mr. A. W. 

 Brown, St. Andrews.* In Table VI. the numbers of males and females in 

 30 catches, representing each month of the period February to 

 September inclusive, are recorded. A distinction has been made in 

 the case of the creel crabs between crabs of legal size, 4^ inches, and 

 those under that minimum. The lists of undersized crabs are not, 

 however, likely to be so exact as those of the gauge crabs, since in 

 some cases, for example at Crail, they are often thrown overboard at 

 sea. At Dunbar, with few exceptions, the fishermen were asked to 

 bring ashore every crab of the catch which was to be enumerated. 



The numbers of crabs collected each month on the beach are detailed 

 in Table IV. ; the hard and soft crabs are separately recorded. Of a 

 total of 2332 crabs (for the sizes of which see Table VIII.) collected on 

 the beach, 1154 were males and 1178 females. This shows a slight 

 preponderance of females. To every 100 females there were 98 males. 

 The crabs were collected without any selection either as to sex or size, 

 and, so far as the total number warrants, may probably be regarded as a 

 fair sample of the crabs which make the beach their habitat. 



The creel crabs (hard and soft taken together) were recorded in two 

 divisions, gauge and under-gauge crabs. In the 20 Dunbar catches 

 there were of the under-gauge crabs, 909 males and 623 females, i.e., 

 for every 100 females 146 males; while at Crail, where only 390 



* By kind permission of Professor M'Intosh. 



