90 



THE GEOLOGIST. 



and ribs become not only actually, but relatively much more numerous as the sbell 

 enlarges ; they are crossed horizontally by a number of (about ten) fine spiral strife. 

 The base makes a right angle with the spire, as in many recent species it is prolonged 

 much in front of the side of the whorl. 



This form nearly resembles C. ornatissimum, Desh., and is possibly 

 its lineal representative. The differences of ornamentation are such as 

 constantly are produced by the conditions of existence. The unequal 

 spiral rows of tubercles of C. ornatissimum become in this form equal, 

 and pass round the spire contiguous, so as to conceal the suture. In 

 this form the excessive multiplication of the elongated tubercles of 

 the anterior row tends in the last whorl to make that row less dis- 

 tinct ; a similar character is also to be noticed in the adult of the 

 French species. The longitudinal ribs of this variety exist in the 

 species from the Gault only as stria?. The basal prolongation of the 

 lip is, however, a feature in the chalk-marl form which will keep the 

 two varieties from being confounded. 



A second slight variation is met with not rarely in the lower argil- 

 laceous chalk of Burwell, in Cambridgeshire. It more closely resem- 

 bles the foreign type than the Brighton specimen. In it the posterior 

 row of tubercles is placed on an elevated rib ; while the anterior row, 

 at least on the anterior half of the shell, is wanting, being represented 

 by rather elongated thickenings of anterior ends of the very fine 

 striae which replace the longitudinal ribs. — {Woodwardian Museum.) 



Cerithitjm. 



A very elongated cone, three times as high as wide. Rather few whorled; whorls flat, more 

 than twice as wide as high. Each whorl is ornamented with three spiral rows of rather 

 small, sharp tubercles, about twelve to fourteen on a whorl in each row. The tubercles 

 are so placed as to form rows nearly perpendicular, or rather parallel to the labial side of 

 the spire. Throughout the shell the spiral rows are at equal distances apart. 



Cerithium (xallicum (D' Orb.) , var. 



A greatly elongated cone, three times as high as wide, many- (about 12-) whorled; 

 whorls inflated, with an elevated mesial angle, from which they become smaller anteriorly 

 and posteriorly, towards the sutures ; the posterior half of each whorl is flat, but the 

 anterior half is slightly convex. The posterior sutural margin is bordered by an elevated 

 rib, which bears a numerous row of closely-placed tubercles. The part of the whorl 

 between the suture and the suture-like anterior margin of this rib is nearly four times as 

 wide as high ; the whorl is one-third as high as wide. A row of sharp, elevated tubercles 

 surmounts the mesial angle of the whorl. The whole spire is finely cancellated, the 

 longitudinal strise being stronger than those transverse. The base is convex, and sepa- 

 rated from the whorl by a slight angular inflation. — {Woodwardian Museum.) 



This differs from the French form chiefly in the small size and 

 number of tubercles on the mesial angle, and is what some authors 

 would regard as a representative species. 



Pleurotoma amphiloga (n. sp.). 



Shell elongated, 5- or 6-whorled ; spire much elevated, two and a half times as high 

 as wide, turreted ; whorls elevated, nearly as high as wide, sides parallel with the sutural 

 shoulder rounded. The whorls are ornamented with moderately elevated, narrow, longi- 

 tudinal ribs, which increase in number, though not in size or closeness, with the growth 



