34 BRITISH OOLITIC AND LIAS1C BRACHIOPODA. 



and extending to near the frontal margin of the valve. Surface finely punctuated ; length 

 and width from 9 to 10 lines, depth 5. 



Obs. This species is generally distinguished from T. resupinata and carinata by its 

 more circular form, the depression in the anterior portion of the smaller valve is less deep 

 and relatively broader, and more regularly curved ; the lateral ridges of the beak are not 

 extended so much in the longitudinal direction, the beak being rounder and not much 

 compressed. Such are the distinctions observable in well characterised specimens of Ter. 

 impressa found abundantly in the Oxford Clay of several parts of England, as at St. Ives, 

 in Huntingdonshire ; and in similar beds near Boulogne-sur-mer. We also find in the 

 Inferior Oolitic beds of Cheltenham, a shell connecting Ter. impressa with T. resupinata, but 

 which, from the size of its foramen and more circular shape, is more properly associated with 

 Ter. impressa ; and I perceive in the ' Prodrome,' M. D'Orbigny places this last-named 

 species in the Inferior Oolite, or his Terrain Bajocien, but omits it in the Oxford Clay, where 

 the type form of the species occurs. The similarity of some of the specimens from 

 Cheltenham to those of the Oxford Clay is so striking, that I would not even venture to 

 give this variety a distinctive name, as some specimens of Ter. impressa may be seen in 

 PI. IV, fig. 9, to have a lengthened shape instead of a short circular form peculiar to the 

 generality of the impressa tribe. In Germany this shell is so abundant that local geologists 

 have distinguished the bed containing them by the name of Impressa-ihorn : here likewise 

 we find a small difference in the aspect of the shell, which seems to me owing more to local 

 causes than to any specific differences. These German shells are not commonly quite so 

 deep ; the smaller valve is a little less convex posteriorly than what is usually seen in those 

 from the Oxford Clay. 



Another small race, or variety, PI. X, fig. 7, is found in the Inferior Oolite of Sherborne, 

 Dorsetshire, which links Ter. resupinata to true T. impressa ; we place it with the last- 

 named species from the characters of its beak ; as none of the species related to the 

 T. resupinata group, hitherto found in the Oolites, have that small laterally pinched and 

 recurved beak, or diminutive foramen so peculiar to the liasic species ; a character any 

 one would perceive who had observed a series of specimens from the two deposits. 

 I have come to the above conclusion after a thorough examination of many hundreds of 

 specimens, nor should I think myself justified in distinguishing these varieties by distinct 

 names. 



Ter. impressa is abundantly found in Germany, especially in Wurtemberg. 



Plate IV, figs. 9 and 10, are type specimens of the species from the Oxford clay, in 



possession of Mr. Morris. 

 ,, fig. 8, var. from the Inferior Oolite of Cheltenham, from a specimen in Dr. 



Wright's Collection. 

 Plate X, fig. 7, a var. from the Inferior Oolite of Sherborne, in the Collection of the 



British Museum. 

 ,, fig. la b c are enlarged illustrations. 



