MB. E. T. niGGINS ON THE OTOLITES OF EISII. 163 



denticulated at the margins, as a general rule wider anteriorly 

 than posteriorly, though sometimes just the reverse — usually 

 longer than broad, ovate, quadrate, pyriform, tapering at both 

 ends, farcate at both ends, or furcate at one end and rounded 

 at the other. In fact, to give anything like an idea of their 

 protean forms, a description of almost every species would be 

 necessary. 



In the Sturionidse, or Sturgeons, which are always arranged 

 between the cartilaginous and osseous fish, the otolites, as might 

 be expected, are of an intermediate type and distinct form, con- 

 sisting of slightly adherent crystalline granules, imbedded in a 

 dense cartilaginous membrane. 



In the true cartilaginous fish the otolites are soft, shapeless 

 masses, closely resembling wet chalk. Under a microscope they 

 are seen to be composed of almost perfect rhombic crystals. 



A good deal of stress has been laid on the shape of the " groove 

 on the under surface of the central otolite, as absolutely necessary 

 for the identification of a species." I am quite ready to admit 

 that there is a very great difference in its shape amongst genera ; 

 but in closely allied species it is so similar as to be almost identical, 

 and therefore, per se, not to be depended upon. So far as my ex- 

 perience goes, identity of outline is the only certain character : and 

 fortunate it is tliat such is the case ; for in by far the largest num- 

 ber of fossil otolites the convex under surface is more or less 

 bouldered and consequently the shape of the groove altered, so 

 that, were this essential for identification, but few species could be 

 recognized. The concavity of the upper surface being better pre- 

 served, the task is rendered comparatively easy, and but Little 

 skill is necessary in dividing them into species. Without attempt- 

 ing to give a rigorous definition of what ought to constitute a 

 species, I may remark in general terms that wherever I find fishes 

 differing from each other to that degree and in those external cha- 

 racters which are usually accepted by naturalists as entitling them 

 to rank as a distinct species, there I find each of these species 

 having a distinct otolite, and with the distinction sufficiently well 

 marked to enable me to refer each otolite, when detached, to the 

 species offish from which it has been taken- — after having, of course, 

 once seen it i7i situ. Now this is, perhaps, more than can be said 

 of either teeth or scales, and certainly than can be stated of any 

 other isolated portion of the skeleton or hard parts in fishes ; and 

 it is a generalization of tlie highest interest in connexion witli 

 palseontological researches, as these otolites are often met with in 



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