9* 



Gigantic Species of Terebrukula 



by's specimens were obtained (by Mrs. Murchison), this is 

 certainly by no means an abundant fossil ; the other observ- 

 ation with respect to it is perfectly correct, as I have never 

 met with it there but as a solitary valve. I believe, however, 

 that this shell, in common with many others, known as 

 " crag species/' has been introduced into that deposit at the 

 expense of an older formation, by the operation of causes 

 analogous to those, which are now indiscriminately mingling 

 the Mollusca of the crag with those of the German Ocean. 

 It is only in the beds beneath the crag that this Tere- 

 bratula occurs naturally grouped, and, when found there, 

 the valves are most commonly in contact, but we should 

 naturally expect to find them dislocated when washed out of 

 their original matrix, either by the encroachments of the sea, 

 or by the action of a river upon the bed in which they may 

 have been deposited. om ri9V9 io biidi sno yd fi "io noto'ioq 



Sowerby designated this species by the term variabilis, 

 from observing that many specimens of it were much more 

 orbicular than others ; now, this variation in shape, constantly 

 exhibited by immature shells, is shown only in a very slight 

 degree by such as have attained the size of/%. 13. a. 



There is one distinction between the young and adult 

 shells deserving particular notice ; it consists in the pro- 

 duction of the margin of the dorsal valve of the latter. 

 During the early stages of growth, the edges of the valves 

 do not encroach upon one another, there being only a simple 

 adaptation of the margins in an even line, sometimes accidentally 

 distorted, from the excessive thinness of the shells at the line 

 of junction. When, however, the shell has attained the length 

 of 3 in. or more, the front edge is rather suddenly produced, 



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13. a, showing the production of the front margin, 



with an abrupt termination asaty%. 14. which is received into 

 a notch in the opposite valve. This locking together of the 



