I. Description of a Great Part of a Jaw with the Teeth of Strophodus Medius, Ow., from the Oolite of Caen in Normandy

1869 Geological Magazine  
PLATE VII.) T HAVE not hitherto seen any specimen so satisfactorily and finely _L illustrative of the affinity of Strophodus to Cestracion as that figured in Plate VIT. and which is now in the British Museum. It consists of the major part of the dental covering of a jaw, including the posterior part of the symphysis, and shows that the principal or largest crushing teeth are in two rows, in each ramus, the hinder one the largest, as in Cestracion. These are followed by two rows (at least) of
more » ... ller crushing teeth, and are preceded by rows of teeth both smaller and more produced at the middle of their working surface, and in the game degree changing from the crushing molar to the conical prehensile type. This dental coating or armature is imbedded in a block of the fine Oolitic building-stone from Caen, which has taken the place of the dissolved cartilaginous support of the teeth, so as to maintain and exhibit the curve of the arch (Fig. la) by which the teeth obliquely overspanned the jaw to which they were originally attached. Of the principal row of teeth (a), six are preserved entire on one side, and the basis of seven on the opposite side of the jaw : the hinder half of this series has been broken off; the fracture of the supporting matrix there demonstrating ; the curve of the convexity of the jaw to which they were originally attached. Seven (Fig. la, 1-7) is thus shown to be the normal.number of these large crushing teeth, which succeed each other from within, outward, and forward : it is the shell of the innermost and last formed which is wanting on the left side of this jaw. • The second tooth, counting from behind, on this side (a2), with a grinding surface 0-035 m. m. in length, 0:013 m. m. in breadth, has that surface moderately convex trans versely, with the convexity highesjt toward the fore end, in the. longitudinal direction: the outer and inner borders straight and parallel; r the fore and hinder borders curved, but. so as to indicate a low angle, fitting the interspace of the correspondingly shaped ends of the two teeth of the'contiguous row. The mam part of the .crown, is sculptured by an extremely fine network of thin ganoin, the meshes simulating pores; but toward the hinder slope the threads run together to form fine sub-VOL. VI.-MO. LIX. 1 3
doi:10.1017/s0016756800159035 fatcat:awr4k54p2ng6ljj44hpjtnwxwe