from the Coralline Crag. 95 



valves is not, as in most instances, attended with a depression 

 of the produced portion in the one, and a corresponding ele- 

 vation in the other, interrupting the regularity of the oval 

 form exhibited by the two shells when in contact. The 

 dorsal, or perforated, valve is the more convex, and its apex, 

 or beak, is produced, and in young specimens considerably 

 incurved. The foramen for the passage of the byssus is 

 always circular, and sometimes large enough to admit the tip 

 of the little finger. In adult shells the length, measuring 

 from the perforation to the opposite margin, is about one 

 fourth greater than the width. The lines of growth are very 

 clearly defined, the shell being in other respects perfectly 

 smooth. The interior of the dorsal valve is thickened towards 

 the apex in a most remarkable manner, and to an extent 

 which sometimes diminishes the capacity of a considerable 

 portion of it by one third or even more. The singular ap- 

 pendages attached to the ventral, or lower, valve of this genus, 

 are often preserved unbroken; but, from their extreme delicacy, 

 it is almost impossible to remove the substance which has filled 

 the cavity of the shell, without effecting their destruction. 



The Terebratula is a particularly interesting genus to the 

 geologist, from the vast numbers in which it is universally 

 found throughout nearly all the secondary formations, and in 

 the more ancient fossiliferous strata ; while a considerable 

 number of species are still in existence, though these latter 

 bear but a small numerical proportion to the amount known 

 only in the fossil state. 



So far as the researches of geologists have at present ex- 

 tended, it would seem that this genus ceased to be abundant 

 at or prior to the commencement of the tertiary epoch. I 

 believe that no instances are on record of the occurrence of 

 any considerable number of species in a deposit belonging to 

 the supra-cretaceous group. The extensive destruction of 

 species which in this instance took place over an immense 

 area, apparently at the same period, will hardly be referred 

 to the agency of any of the causes now in operation by which 

 extermination is thought to be effected ; more especially, too, 

 as the living shells are found at various depths, and in the 

 seas of nearly all climates. Terebratula variabilis is, I 

 think, the only tertiary species yet described as occurring 

 in this country, though three o_r four are said to exist in the 

 British seas. A small, compressed, longitudinally striated 

 species is abundant in the London clay of the Isle of Sheppy ; 

 and, besides the one now figured, Mr. Searles Wood possesses 

 two others from the coralline crag, neither of which is known 

 in the living state. 



ii 4- 



