of the Transition Formations of the Rhine, 163 



Listeri is tlie sharp ridge^ which must be regarded as the hmit 

 between the broad back and side, and wliich intersects the 

 lateral saddle at the centre. The ridges of the shell are strong 

 and simple on the inner whorls ; on the outer they become 

 finer and finer, and are bifurcated at the commencement of 

 the back. They are always perceptible as ribs even upon the 

 casts. The folds are raised upon the dorsal ridge in the form 

 of scales, so that it appears indented ; and this is well de- 

 scribed by the series marginalis tuherculorum that Martin 

 gives in his characteristic. The number of ridges upon the 

 third or fourth whorl is about thirty or forty. Their num- 

 ber rapidly augments in the last whorls. A remarkable and 

 very interesting fact is, that upon the outer whorls the folds 

 change their direction altogether. Upon the inner whorls, 

 contrary to the general rule for Goniatites, they form a curve 

 upon the back, of which the convexity is directed forward. 

 This curve becomes more and more flattened, the folds keep 

 for some time in a straight line, and at last resume the or- 

 dinary direction. The change in the direction of the folds is 

 figured, PL II. fig. 8. It proves that no great importance 

 should be attached to the direction of the striae or folds of the 

 shell in separating the Goniatites from the rest of the Am- 

 monites. 



17. Ammonites T)iadema, Goldf. PL II. fig. 5, 6, 7- 

 The dorsal lobe is nearly equal in breadth and height. The 

 middle elevation is rounded like the dorsal saddle. The late- 

 ral lobe is almost as deep as the dorsal lobe and somewhat 

 sloped in the form of a tongue. The lateral saddle is broad 

 and rounded, and declines gradually tow ards the suture. The 

 increase in height is 3*52 ; the increase in breadth 0*62 to 0*65. 

 The inner whorls are almost entirely enveloped, leaving a nar- 

 row and deep umbilicus. The form is rather discoid than glo- 

 bular ; the shell finely folded. The name which Goldfuss has 

 chosen for this ammonite may have been occasioned by the 

 smooth furrows, which to the number of four or six upon the 

 circumference of a whorl, run along on the back and side, at 

 nearly equal distances. The direction of these furrows en- 

 tirely follovrs that of the plications of the shell, notwithstand- 



X 2 



