BIVALVIA. 185 



procured in the Arctic seas. In a comparison with the recent shell, some slight 

 difference may be observed, the Crag one being somewhat compressed, with a rather 

 more elevated umbo ; in the two specimens belonging to Mr. Morris, there is a dif- 

 ference, one being less transverse than the other, and the smaller of the two has the 

 margin of the valves deeply crenulated, while in the other it is quite smooth, and in 

 both the umbo is much eroded, thereby considerably reducing its elevation, but in the 

 Crag shell the umbo has not in the least been acted upon in that manner. There is a 

 considerable resemblance with A. (jracilis which is also found in the Red Crag, but 

 that species is more equilateral, with the sculpture more distinct and elaborate. 



12. AsTAKTE GRACILIS, Mitnster. Tab. XVII, fig. 3 a—e. 



AsTAETE GRACILIS. Miliist. apud Goldf. Pet. Germ., vol. ii, p. 194, t. 135, fig. 4a~c. 



— PROPiNQUA. Id. „ vol. ii, p. 194, t. 135, fig. 3 a — c. 



— Galeotti. Nijst. Coq. Foss. de Belg., p. 159, pi. vi, fig. 17 a — c, 



— LIRATA. Conrad. Sec. Lyell Proc. Geol. Soc, 1845, p. 555. 



Spec. Char. Testa orhiculato-trigonuld, ovatd, transversa, tumidd aut compressiusculd 

 plurimum subaquilaterali ; lineatd vel sidcatd, lineis magnis convexis ; margine crenulato 

 vel integro. 



Shell roundedly trigonal, ovate, transverse, tumid, or compressed, for the most 

 part nearly equilateral, ridged or sulcated externally, with large rounded lines ; margin 

 crenulated or smooth. 



Diameter, -|ths of an inch. 



Localities. Cor. Crag, Sutton, Ramsholt, Sudbourn, and Gedgrave. 

 Red Crag, Sutton, Bawdsey. 



This species is very abundant in the Coralline, while a few specimens only, and 

 those not in very good condition, have been found by myself in the Red Crag. The 

 most common variety is that which corresponds with the figure by Goldfuss ; this is 

 very regular in shape, nearly equilateral, longer than high, and covered upon the ex- 

 terior with about forty or fifty rounded, regularly-formed ridges, wider than the spaces 

 between them upon the body or older part of the shell, while in the younger or in the 

 umbonal region these ridges are somewhat sharp upon the hinder or upper side, or, as 

 it were, obtusely reflected ; the same character in regard to the ridges pervades all the 

 varieties, and covers the entire surface, except the lunule and corselet, which are 

 smooth ; another form is more trigonal, has a thicker hinge, with a more prominent 

 umbo, and is quite as high as it is long; var. midtilineata, (fig. 3 e,) of which I have only 

 a few specimens, varies in no other character than a greater number of these con- 

 centric ridges, amounting in th< se to as many as 90 ; the shell is very thick, tumid, 

 and elevated, and is as high as it is long. In the full grown state the shell is crenulated 

 at the margin, and although large specimens are often met with in which this part is 

 quite smooth, these have not, it is presumed, attained to full maturity. It was probably 



