TEREBRATULA. 



27 



always truncated by an apicial, emarginate, or entire foramen, of a circular or elongated 

 shape, variable in its dimensions, and more or less separated from the umbo by a triangular 

 deltidium in one or two pieces, the foramen being surrounded by the substance of the 

 larger valve and deltidium ; no true area ; lateral beak, ridges undistinct, short, recurved 

 to join the hinge margin, or continued along the sides of the shell without recurving, in 

 which last case there commonly exists between it and the hinge a flatness or false area. 

 Smaller or imperforated valve convex or concave, with or without a longitudinal mesial 

 fold ; hinge composed of two teeth in the larger valve, which articulate with corresponding 

 sockets in smaller one, so that the valves cannot be separated without fracturing one of the 

 teeth ; internal ribbon-shaped lamella, (partly supporting the ciliated arms,) attached only 

 to crura, short or elongated, and more or less folded back on itself ; animal fixed to sub- 

 marine bodies, by muscular fibres passing through the foramen, structure perforated. 



05s. It has been proposed to divide the genus Terebratula according to the length of 

 the loop, which will be referred to in our Introduction. 



We have admitted forty species of British Oolitic Terebratula, which can, we believe, 

 be conveniently retained, although, strictly speaking, many species pass one into another, 

 by some one or more common characters. 



In the following table we have arranged these species into two sections, those with a 

 loop extending to near the frontal margin, and those in which the process does not attain, 

 or exceed, half the length of the shell ; no external characters, however, seem to denote if 

 the loop be long or short, and we have placed a point of interrogation before those species 

 in which we have not actually seen it, but where, from observations or indication, we 

 believe it to be so. 



1st Section. 



Loop simply attached 

 to crura, and extending \ 

 to near the frontal 

 margin. 



Beak laterally compressed, 

 carinated ; beak ridges 

 continued along the sides 

 •without recurving to join 

 the hinge margin ; sur- 

 face smooth. 



Beak uncompressed late- 

 rally ; beak ridges soon 

 becoming indistinct, or 

 recurved to join the hinge 

 margin ; surface smooth. 



Surface plicated. 



f Terebratula quadrifida. 



{ 



{ = 



? 



cornuta. 



Edwardsii. 



Waterhousii. 



resupinata. 



Moorei. 



impressa. 



carinata. 



emarginata. 



Waltonii. 



numismalis ; var. subnumismalis. 



Bakerise. 



digona. 



obovata. 



ornithocephala. 



lagenalis. 



sublagenalis. 



eardrum. 



