UPPER, OR FLINTY CHALK. 209 



The length of this species exceeds its width. It is distinguished from 

 the two former, by the straightness of the front, and the deep undulations 

 on each side. The beak is produced, and its perforation large; both 

 valves are equally gibbous. It sometimes attains 1-5 inch in length. 



Localities. Common in every quarry in the South Downs. 



The three species above described are easily distinguished from each 

 other ; but it must be confessed, that the two following are so nearly re- 

 lated to the preceding, and in a suite of specimens pass so insensibly into 

 each other, that the propriety of their separation may be questioned. I 

 have, however, followed the arrangement of Mr. Sowerby, to avoid a mul- 

 tiplication of synonymes*. 



79. Terebratula i?itermedia. Min. Conch. Tab. xv. fie. 8. 



" Obscurely five sided, rather depressed, smooth; larger valve most 

 convex ; front margin undulated ; three depressions in the smaller valve, 

 and two in the larger." 



The undulations in this species are not confined to the margin, but 

 extend some distance along the body of the valves ; the front is depressed. 



Localities. Upper, and Lower chalk, South Downs. 



80. Terebratula sejniglobosa. Min. Conch. Tab. xv. fig. 9. 



" Nearly circular, gibbous, smooth; larger valve deepest, and uni- 

 formly gibbous ; front margin undulated, with two ridges on the lesser 

 valve." 



The width of this shell nearly equals its length ; some specimens are 

 very gibbous. The undulation in front, the two eminences on the upper 



* A friend who has paid some attention to the subject, has favoured me with the following 

 arrangement of these sheUs. 



Terebratula undata. Min. Conch. Tab. xv. figs. 7, 8, 9. 



Spec. char. Obovate, both valves convex, smooth; margin of the front either straight or 

 depressed, undulated on each side ; beak produced. 



Var. a. suhundata, (fig. 7.) Longer than wide, front straight; valves equally convex. 



Var. h. intermedia (fig. 8.) Rather depressed, longer than wide; larger valve most convex ; 

 front undulated; undulations extending upon the valves. 



Var. c. semiglobosa (fig. 9.) Subglobose; length, width, and thickness nearly equal; front 

 margin undulated. 



The above descriptions refer to those specimens which present the most striking differ- 

 ences i it would be easy to select examples of every intermediate gradation of form. 



