210 UPPER, OR FLINTY CHALK. 



valve, and the width and thickness of the united valves, appear to be the 

 distinguishing characters of the species. 



The shells of the terebratulas above described, are invariably changed 

 into crystallized carbonate of lime ; their cavities, Hke those of the echi- 

 nites, being frequently Hned with calcareous spar, or crystals of sulphuret 

 of iron. Their prevailing hue is a deep cream colour, but sometimes 

 shades of red, brown, and blue, are observable ; may not this in some 

 measure depend on the colour of the original ? 



LocaHties. Upper, and Lower chalk. South Downs. 



81. Terebratula jo/ica^zVis. Min. Conch. Tab. cxviii. fig. 1. 



— var. b. octoplicata. Min. Conch. Tab. cxviii. 



fig. 2. 



var. c. concinna. Mm. Conch. Tab. Ixxxiii. 



fig. 6. 



Gibbous, transversely obovate, longitudinally striated: margin finely 

 serrated ; front sinuate, elevated with from six to twelve acute plicae ; 

 beak slightly projecting. 



The difference in the number of folds on the front, is assumed by Mr. 

 Sowerby, as the specific distinction between his T. jjlicatilis, and T. octo- 

 plicata; T. concinna is separated on account of its more globose form. 

 The specimens in my possession vary so much in the number of plic«, and 

 in the convexity of the valves ; and the characters of each are so in- 

 timately blended in many examples, that I have been compelled to con- 

 sider them as only varieties of the same species. 



In var. a. the length of the shell exceeds the depth of the united 

 valves, and the width is one-third greater than the length; the front 

 margin is elevated by a broad sinus containing twelve plicae. Var. b. 

 has but eight plicse. In var. c. there are but seven plicae ; the valves are 

 globose, the depth and width of the shell being nearly equal. 



The strise are rounded, and diverge with a gentle sweep from the beaks 

 to the margin ; the number on each valve varying from thirty to fifty. 

 The lower valve is less convex than the upper ; the beak is but slightly 

 produced, and the perforation very small. The lateral margin is acutely 



