350 FOSSILS FROM THE 



day amonsj the rocks of Thurso in disintemng some of these 

 very specimens, it occuri'ed to me to inquire whether trie 

 misplacement of the thoracic cincture, — now for the firsc 

 time apparent among the rocks, — might not have a simpler 

 as well as a more complex reading, like those chapters of the 

 sacred volume that, while charged with high mystery for the 

 future to read, had also their plain, easily-understood lessons 

 for the men of the age in which they were first promulgated. 

 What, I asked, may be the simpler meaning of this strange 

 aberration, now so prevalent in the ichthyic kingdom ? and 

 my thoughts in reply to the query arranged themselves thus : 

 — In fishes in which the gills are fixed, and the gill-openings 

 small and numerous, the common action of the muscles, un- 

 assisted by any extraordinary mechanism, seems sufficient to 

 carry on the work of respiration, and to regulate the open- 

 ing and shutting of the sluices through which the water must 

 be expired, but through which it must not be inhaled. But 

 when the openings are restricted to two, and are compara- 

 tively of large size, we find that the important mechanism of 

 a gill-cover is required, and of a strong well- fitted band of 

 bone on which the cover may tightly fasten down. In order 

 to shut a box firmly, and with precision, it is not only neces- 

 sary that its lid and sides should be well fitted, but that 

 they should also possess considerable rigidity of substance. 

 And this rigidity of substance the gill-cover — or box-lid of 

 my illustration — always possesses. It is usually composed, 

 in the earlier ganoids, of one, two, or more plates of enamel- 

 led bone, completely united and delicately hinged. But how 

 impart the necessary rigidity to these soft abdominal integu- 

 ments, on which, from their latero-ventral position, the gill- 

 covers must necessarily rest ? Without some means of im- 

 parting rigidity to these (the sides of the box), the rigidity 

 of the lid would be of no avail ; the gill-openings would ad- 

 mit the water; and the fish, when exposed to powerful 



