414 [May 1, 



Thickness and Description of Strata. 



« Thickness 



No. in feet. 



4. Perna Mulleti bed, with numerous Gryphoia, Ostrece, &c. &c. very 



fossiliferous - - - - - . -g 



3. Clay, very fossiliferous, containing layers offish-bones, teeth, &c. but 



regular - - - - - - - .3 



( Wealden. ) 



In the accompanying Table are given the ranges of such of the 

 fo.ssils of the above strata as were collected and noted by the au- 

 thors on the spot. 



5. Description of the Mouth of a Hybodus found by Mr. Bos- 

 CAWEN Ibbetson in the Isle of Wight. By Sir Philip 

 Malpas de Grex Egerton. Bart. M.P., F.R.S., F.G.S. 



The present memoir is the result of the examination of an Ich- 

 thyolite discovered by Mr. Boscawen Ibbetson in the Isle of 

 Wight, near the junction of the Lower Green-sand with the 

 Wealden, and sent to me in the hopes that it might tend to show 

 to which of the two formations this bed should be assigned. The 

 evidence it affords on this question is neither direct nor conclusive, 

 inasmuch as it is an undescribed species, and consequently any 

 deductions beyond those based upon general affinities would be 

 unwarrantable. In another point of view, however, this specimen 

 is of high scientific value, as it sets at rest the long-mooted ques- 

 tions of the i-elative characters of the upper and lower teeth, and 

 their general contour in the individuals composing the genus Hy- 

 hodus so extensively occurring in the secondary strata. Mr. Ibbet- 

 son has had the rare fortune to bring to light the entire mouth of 

 a fish of this genus. The left side is slightly crushed, but the 

 other retains its natural form, and the greater portion of the teeth 

 in both the upper and the lower jaw. The former measures 10 inches, 

 and appears to have carried twenty -four teeth in the front series; 

 the latter measures 7^ inches, and has nineteen teeth in series, one 

 on the symphysis and nine on either side. Two rows of succession 

 teeth are traceable behind the front series. The mouth is slightly 

 open, and when seen in profile is more arcuate than in the recent 

 sharks. The upper jaw has a broad notch for the reception of the 

 thickened symphysis of the lower mandible. The teeth have a 

 central cusp, rather hooked, and two secondary cusps on either 

 side ; the enamel is strongly plicated ; the teeth only recently 

 brought into use have the plicas extending to the apex. The bases 

 are wide, and have the rugose character so generally found in this 

 genus. The lateral teeth present the same characters as the more 

 central ones, but are rather smaller near the angle of the jaw. 



