MEGATHYRIS. 361 



presence abundantly in a sedimentary deposit should lead 

 to such an inference respecting the circumstances under 

 which the stratum was formed. 



M. cisTELLULA, Scarlcs Wood. 



Minute, not ribbed ; truncated at the umbones. 



Plate LVII. fig 9. (magnified). 



Terebratula cistellula, Searles Wood, Annals Nat. Hist. vol. vi. p. 253 

 (fossil). 

 „ seminulum ? Jeffreys, Ann. Nat. Hist. vol. xix. p. 31"2. 



The extreme minuteness, and the scarcity of specimens, 

 of this the smallest of existing Terebratuldt has prevented 

 us from giving so detailed an account of the species as 

 we could have wished. 



The shape, even in the few examples we possess, has a 

 remarkably wide range of variation. That which we have 

 figured, and which has a greater appearance of regularity 

 than the majority of individuals, has its contour almost 

 hemispherical, but the figure of certain examples is assuredly 

 much more orbicular, but always more or less truncated 

 above. The valves are rather depressed, and seem nearly 

 equal to each other in convexity and extent ; they are a 

 little pellucid, tolerably strong for their size, are devoid 

 of radiating striae, having only some concentric lines of 

 growth, and are wholly or partially covered with minute 

 raised dots. Their colour is tawny-yellow or pale yel- 

 lowish-brown ; but this hue may possibly be caused by 

 the presence of the dried animal. The hinge line is 

 straight and much produced ; the central retusion of the 

 opposite margin is shallow but distinct. The mesial groove 

 of the lower or ventral valve is well marked, and on either 



VOL. II. 8 A 



