Correspondence. 275 



constructed of Vertebrata. Cuvier talks much of the Acanthopterygii 

 being the most homogeneous in their variations; but are not the 

 groups of Sharks and Cyclostomi quite as homogeneous ? Nay, are not 

 Fistularia and Vomer more distinct from each other in form than a 

 Shark from a Skate, or a Lamprey from a Myxine? 



There are some relations that require still to be expressed by my 

 foregoing arrangement, such for instance as that of Platycephalus to 

 Eleotris, as that of Sphyrsena to certain Esocidse, &c. &c. Are all such 

 merely relations of analogy ? If so, they are expressed ; but I cannot 

 help thinking, that the relation is still stronger than that of mere 

 representation. 



All geological forms may I think be referred with ease to the fore- 

 going arrangement, even the most anomalous in appearance, such as 

 Jphalaspis ; for this fossil form may in my opinion be understood by 

 looking at the head of Platycephalus. However the most extraordinary 

 forms of fossil fish belong to the Ganoids of Agassiz, or rather to the 

 Sturiones, and those other orders of the class Pisces, that present the 

 fewest existing forms. But on this head I shall at once frankly say, 

 that if any fossil forms can be shewn not to fall into a place in the 

 preceding arrangement, then my general view of Ichthyological affini- 

 ties is wrong ; for I am convinced that there is but one system for all 

 animals, whether Antediluvian or not. I shall write you on Echinidse in 

 my next, and send you some the very first opportunity. 



Description of an Instrument for measuring the hardness of Minerals, extract- 

 ed from a letter of Lieut. R. B. Smith, Esq., Bengal Engineers, dated 

 Allahabad, 15th January, 1841. 



"I have long wished to find some easier method of determining the 

 hardness of mineral bodies than that furnished by Professor Moh's scale. 

 I have recently amused myself by constructing a little apparatus, which 

 I fancy will answer the purpose. I give 

 you a rude sketch of it, from which I 

 dare say you will easily see the principle. 

 The frame work is of wood. The centre 

 pin is of hard steel sharpened at the |j nia 



point, working in a screw socket in the wood, so as to admit of ready 

 adjustment. When the instrument i3 to be used, the substance is placed 

 under the steel pin, and certain weights (1, 2, 3,) are placed on the top 

 of the frame. The loaded frame is then drawn over the mineral, and if 

 the weight is sufficient to cause the pin to scratch its surface, its hard- 



