254 The Naturalist of Cumbrae. 



only two semi-circular canals, and in the myxine there 

 is only one of these canals, but in all other fishes 

 there are three, as in higher animals ; they communi- 

 cate with a vestibule, in which are contained the bony 

 plates called otolites ; there are usually two of these 

 flattened, somewhat oval organs, and one is larger 

 than the other. But in a good many fishes, such as 

 the plectognathi and lophobranchiates, the otolites 

 are represented by calcareous dust. No fish possesses 

 a cochlea, or a true tympanic membrane, but some- 

 times there is a connection between the labyrinth of 

 the ear and the air-bladder made by a chain of small 

 bones. In the roach the air-bladder is exceedingly 

 small, extending under only two vertebrae, and is 

 united with the head in this way. The external ears 

 of the skate are on the top of the head." * 



In the month following his visit to Montrose, 

 Robertson went with his friend Dr. G. S. Brady to 

 the Norfolk broads in quest of fresh-water ostracoda. 

 In working at the sides of the broads and along the 

 banks of rivers and canals the little dredge was in 

 constant requisition. It could be thrown a consider- 

 able distance into the water and brought back full of 

 the desired material. To make this portable it had 

 to be passed through a sieve to get rid of the coarser 

 matters, such as stones, and decaying stems, and 

 leaves, the residuum being then washed to clear away 

 as much of the fine mud as possible before bagging 

 it up. 



By the dredge much was obtained that would not 



* Cassell's " Natural History," vol. v. p. 12. For a slightly different 

 description, see the Appendix. 



