384 Dr. W. B. Carpenter on the Minute Structure of 



determination or not, I think no doubt can be entertained, that 

 the presence or absence of these perforations affords a most im- 

 portant character, by w hich the genus Terebrutula may be divided. 

 I should mention, tliat the determination of this character cannot 

 be certainly eflected, in fossil species, without making a section 

 of the shell ; since the perforations are often filled up with stony 

 matter, in such a way as to obliterate the punctations on the 

 surface. 



The diameter of these perforations in Terebrutula is usually 

 about -0006 or -0008 of an inch. In Productu they are larger, being 

 about -0010 or -0012 of an inch ; and they are readily discernible 

 with the naked eye in thin sections or in fractured pieces of the 

 latter. They are readily distinguishable in the decalcified mem- 

 branous basis of the recent species, and are obviously lined by 

 distinct membranous tubes. I am much inclined to believe that 

 these tubes are prolongations of the mantle, which Mr. Owen has 

 observed to possess an unusual adhesion to the shell. 



I cannot but think that the value of the microscope, as an in- 

 strument of geological research, must be at once evident from 

 these statements. The genera Terebratula, Spii'ifer and Producta'^ 

 may be at once distinguished from each other, and from all other 

 shells, by the characters supplied by a fragment of shell, which a 

 pin^s head would cover. 



Whilst upon the structure of the Brachiopoda, I may mention 

 (though rather anticipating my next head), that the genera Lin- 

 gula and Discina [Orbicula) agree with each other, and differ en- 

 tirely from the rest, in a very peculiar set of characters : — their 

 shells possess extremely little calcareous matter, being made up 

 of thin horny plates adherent to each other ; every one of these 

 horny plates, however, is traversed by a set of very minute tubuli 

 running aslant through it, and very closely resembling, both in 

 size and arrangement, the tubuli of dentine, and those which I 

 have observed in the shell of Crustacea. I have nowhere else 

 discovered, among ]Mollusca, a structure which could be mistaken 

 for this. Consequently these two genera are at once separated, 

 by microscopic examination, from all others. 



The tubular variety of membranous shell-structure, as it exists 

 in the shells of certain Lamellibranchiata and Gasteropoda, is the 

 last form which I have at present to describe. The size of these 

 tubuli varies from about 1 -20,000th to l-3500th of an inch ; but 

 their general diameter, in the shells in which they most abound, 

 is about l-6000th of an inch. The direction and distribution of 

 these tubes are extremely various in different shells. In general 

 they form a network which spreads itself out in each layer, nearly 



* J{' 1 am correct in tlie characters I have assigned to them. 



