338 Mr. A. S. Woodward on the 



its occurrence in both rami of two other mandibles in 

 Mr. Leeds's collection ; and the only explanation suggesting 

 itself is that it relates to some attachment of ligaments or 

 muscles specially adapted for the successful wielding of the 

 unusually powerful dentition. 



Dentition. — The teeth, most completely preserved in no. 4, 

 are all detached, and those of both jaws mingled together in 

 such a manner that the task of restoring the original dentition 

 is somewhat difficult. Two or three definite facts, however, 

 seem to form a satisfactory basis for a plausible attempt at a 

 solution of the problem. In the first place, Sir Richard Owen 

 has already determined the exact number and characters of 

 the dental series in one jaw of the so-called Strophodusmedius'^; 

 and the homologous teeth can easily be recognized in the 

 present collection. Secondly, in the jaw just mentioned there 

 is no median symphysial series, whereas among the Oxford 

 Clay teeth there are five examples which must have undoubt- 

 edly occupied such a position ; and it thus becomes probable 

 that, as often in Cestracion f, one jaw had a median symphy- 

 sial row of teeth, while the opposing jaw possessed none. 

 Thirdly, upon a fragment of the right mandibular ramus a 

 few teeth of series 1 and 2 are actually preserved in position ; 

 so that these rows can be identified with complete certainty 

 and by inference also their 0]iposing series. And, lastly, 

 placing the median row of teeth in the lower jaw the teeth of 

 series 3 and 4 can readily be arranged to make the dentition 

 of both jaws thus far of precisely the same extent. The 

 intercalation of the median series and the greater relative 

 length of the teeth of series 4 in the lower jaw precisely com- 

 pensate for the greater relative length of the first three paired 

 series in the upper jaw ; and on this account I venture to 

 think that the following determinations will prove for the 

 most part well founded. Examples of the several teeth of 

 the upper jaw are shown in PI. XII. fig. 2, and a correspond- 

 ing set from the lower jaw in fig. 3. 



Commencing with the upper jaw, six teeth of series 1 are 

 preserved upon the left side and seven upon the right. Each 

 measures 0'025 m. in length and is much elevated, and the high 

 arched crown is longitudinally keeled, narrowest anteriorly, 

 and broadest at the point of its maximum elevation, nearer 



* Geol. Mag. vol. vi. 1869, p. 194, pi. vii. 



t Sir Richard Owen (Geol. Mag. vol. vi. 1869, p. 196) has remarked 

 that in the lower jaw of Cestracion a median symphysial row of teeth is 

 present, while in the upper jaw it is absent. ' So far, however, as the 

 present writer has had the opportmiity of observing, the character is not 

 constant. 



