from the Cambridge Greensand. 229 



correspond with his description and the figure answering for 

 Mayor's MS. species. That form is spoken of as "ornee de 

 cinq k six sillons qui partent de I'ombilic dans la direction 

 qu'auraient des tangentes au cercle ombilical." This character 

 may perhaps not be sufficient to distinguish a species ; but it 

 certainly makes a well-marked form, indicating an animal which 

 formed periodic varices. 



Ammonites latidorsatus, Mich. 

 A. latidorsatus, D'Orb. Paleont. France ; Pictet, Gres Vert, pi. 3. fig. 6. 



Few-whorled, much inflated, whorls two-thirds embracing, a 

 little wider than high, with nearly parallel flat sides, and a 

 rather depressed round back, which rounds into the sides. The 

 body-chamber is half as high again as the whorl at the opposite 

 side of the shell, than which the umbilicus is slightly narrower. 

 The umbilicus is deep, with the flat and almost horizontal ven- 

 trum making a sharp angle with the side, though the line of 

 meeting is just rounded. The cast appears perfectly smooth, 

 but is marked by shallow and narrow sulcations arising in the 

 umbilicus and passing over the back, where two are separated 

 by interspaces narrower than the height of the whorl. The 

 mouth is lunate. 



The septa are like those of A. planulatus, the only difi'erence 

 being that the inferior lateral lobe is not cleft so deeply. 



This is a very rare fossil ; and the only specimen known to 

 me was detected in the collection of the Rev. J. F. Blake, who 

 has presented it to the University Museum. The diameter is 

 1;^ inch, and the last fourth of the outer whorl is devoid of 

 chambers. It corresponds with the Continental figures and 

 description, though this sulcated forn), which cannot be con- 

 sidered typical, is a variety distinguishable from Michelin's 

 smooth shell. 



Ammonites Mayorianus, D'Orb. 



A. planulatus, Sow, Min. Con. j Sharpe, Chalk Moll. pi. 12. fig. 4. 

 A. Mayorianus, D'Orb. Paleont. France, T. C. 

 A. octosulcatus, Sharpe, pi. 19. fig. 3. 

 A. Griffithsii, Sharpe, pi. 11. fig. 3. 



Inflated, with few whorls, more than half embracing ; mouth 

 as wide as high. Sides a little inflated, and passing impercep- 

 tibly into the round back. Umbilicus about as high as the 

 body-whorl, with the horizontal but inflated ventrum rounding 

 into the side. The depth of the ventrum exposed is about the 

 same as the width of the unembraced part of the adjacent whorl, 

 with which it forms a right angle. The cast commonly shows 

 very fine depressed ribs, arising about the middle of the side, 

 and passing over the back; they are close together, and oa 



