Prof. Owen — On a Jaw of Strophodus. 195 



Eight teeth of the foremost or symphysial row of teeth, d, are pre- 

 served on the left side. The exposed parts of the worhing surface 

 augments from 0*008 m. m. — the longitudinal diameter of the inner- 

 most, d, to 0*020 m. m. that of the sixth tooth in advance. The degree 

 of convexity of the grinding surface increases from the innermost to 

 the fourth ; in advance of this, the ganoin coating has been more or 

 less broken away; in the fifth and sixth teeth, the ridge continued 

 from the convexity to the ends of the teeth becomes marked, and 

 more strongly as the tooth advances in position. The right and left 

 teeth of this foremost series are alternate, and loosely interlock at the 

 mid-line of the symphysis. 



The parallel ridged disposition of the ganoin occupies the greatest 

 proportion of the crown in the teeth of the anterior row ; the reti- 

 culate disposition, rather coarser than in the large posterior teeth, is 

 confined to the obtuse summit of the crown. 



Eeturning to the hind part of the dental series, the largest row 

 first described is succeeded by one, e, of very small teeth, with a uni- 

 formly convex grinding surface of an oblong elliptical form, of which 

 the long diameter, in the direction from within outwards, exceeds 

 that from before backwards. The long diameter is 0*011 m. m. ; 

 the short diameter is 0*008 m. m. The reticulate pattern prevails 

 over their surface ; it is coarser or with larger meshes than in the 

 teeth in advance. Of only one tooth in this series is the ganoin of 

 the crown preserved ; parts of five other teeth of this row, however, 

 remain in situ. The indications of a succeeding hinder row, /, of 

 similar, but rather smaller teeth, are obvious ; and the base of one 

 tooth of a third, and probably hindmost row, g, is preserved. 



In comparing the dentition of Strophodus with that of Gestracion, 

 the chief difference is seen in the smaller number of rows anterior to 

 the principal or largest : three rows, in each ramus, occupy the 

 interval between such principal row and the mid-line of the symphy- 

 sis ; in Cestracion seven rows occupy that space in the upper jaw, 

 and nine rows in the lower jaw ; in this jaw, moreover, a medial 

 azygous row occupies the mid-line of the symphysis, which is not 

 the case in the upper jaw. (See Figure, inserted at page 236.) 



According to this analogy, the teeth in the present specimen agree 

 in arrangement with those of the upper jaw of Cestracion ; but in the 

 more gradual diminution of size, as they approach the symphysis, the 

 teeth agree more with those of the lower jaw in Cestracion ; the de- 

 crease being much more abrupt in the series next but one in advance of 

 the principal series in the upper jaw of Cestracion. It is unlikely, 

 from the minor number of rows and the larger relative size of the 

 anterior teeth in Strophodus, that an azygous mesial row should be 

 interposed at the symphysis of the lower jaw. But the elements for 

 absolutely determining whether the present specimen be from the 

 upper or lower jaw are wanting. I have assumed the latter for 

 facility of description, partly from the gradual decrease forward in 

 the size of the teeth, partly because a detached fossil jaw is so much 

 more commonly a lower than an upper one. 



But what is of more interest and importance is, that, in this long 



