i 92 INTRODUCTION TO ZOOLOGY. 



by the retiring tide. Each had a thread-like extremity, buried 

 in the sand to the depth of two or three inches, and highly 

 elastic. We have been unable to ascertain to what kind of 

 Cuttle-fish they belonged.* Mr. R, Ball has recorded, as 

 occurring in the Irish seas, twelve species of Cephalopoda, 

 three of which were previously undescribed.f 



The remains of animals of this family have been found along 

 with the undigested portions of the food of the gigantic saurian 

 reptiles of remote ages ; and thus, in the words of Dr. Buck- 

 land, "the general law of nature,, which bids to eat and be 

 eaten in their turn, is shown to have been co-extensive with 

 animal existence on our globe; the carnivora in each period 

 of the world's history fulfilling their destined office, to check 

 excess in the progress of life, and maintain the balance of 

 creation." 



The brief space devoted to the Mollusca cannot be closed 

 without adverting to their great importance in a geological 

 point of view. Their shells, which, in a fossil state, are found 

 in the secondary rocks, are different from those of any animals 

 of the same tribes now existing. They may belong to the 

 same families, in some cases to the same genera, but invariably 

 the species is extinct. In the older tertiary rocks, we meet, for 

 the first time, with shells in a fossil state, which are specifically 

 identical with some now living. But the number of such is 

 so small, that it has been estimated at only three and a half 

 per cent, of the entire. As we approach the more recent 

 strata, the number of shells of species still living continues to 

 increase, until, in those tertiary rocks which are the most 

 recent, it constitutes nine-tenths of the entire number. Hence 

 shells have, with great propriety, been termed " the medals 

 principally employed by Nature in recording the chronology of 

 past events." J 



An aid in the detection of generic resemblances between 

 different fossil shells, and also between recent and fossil, has 



* They have so much resemblance to the ovisacs contained in the 

 ovary of Rossia palpetrosa, figured by Professor Owen in the appendix 

 to Ross's voyage, that we are inclined to surmise they must have been 

 those of some species of the same genus a conjecture the more probable 

 as to this genus belong two species, added to our Fauna by Mr. Ball. 

 Ovisacs described to us as similar to what we have noticed were found 

 by Miss Ball on Clontarf strand. 



t Proceedings Royal Irish Academy, 10th Jan. 1842. 



j Ly ell's Principles of Geology, vol. i. page 283. 



