AMMONITES. 25 



know nothing certain respecting the sexual differences of form in Ammonites ; but even 

 if it were admitted that such differences existed, they would not suffice to bring 

 A. varians and A. Coiipei together, as in each species there are two well-marked differences 

 of form, so that there are varieties enough to supply distinct sexes in each species. The 

 young shells of both varieties .of A. Coupei nearly resemble each other ; the differences 

 between them begin in their middle stage, and increase as they grow older, the 1st variety 

 gradually approaching A. varians in form, while the 2d variety recedes farther from that 

 species. The young A. varians always present a strong contrast, in its flat, faintly-ribbed 

 form, to the rounded and tuberculated young shell of A. Coupei .• it is only in the adult 

 form of one variety of each species that any confusion can arise between them. 



6. Ammonites cinctus, Mantell. Plate IX, fig. 2. 



Ammonites cinctus, Mantell. Fossils of the South Downs, p. 116. 

 — — Sowerby. Min. Conch., t. 564, fig. I. 



A. testa discoided ; ambulacris rotundatis, transversim costatis ; costis tenuihus, alter - 

 natim majoribus furcatis ambulacrum totum amplectentibus, minor ibus inter mediis dor salibus ; 

 dorso rotundato ; umbilico parvo ; aperturd ovali. 



Shell discoidal, with rounded whorls crossed by numerous slight, irregularly flexuous 

 ribs ; these are twelve in number at the umbihcus, divide into two before reaching the 

 middle of the side, and are continued over the rounded back; between each of the 

 branches of the larger ribs there are one or more short ribs, which only extend across tlie 

 back ; umbilicus small ; whorls about two thirds concealed ; aperture oval. The septa 

 have not been seen. 



Diameter, 3-8 inches; width, 1"1 inch. 



From the Grey Chalk of Middleham, Sussex. The figure is taken from Dr. Mantell's 

 original specimen, now in the British Museum. No other specimen of the species is 

 known. 



A. cinctus is well characterisd by its very peculiar ribbing, and cannot be confounded 

 with any other species. The specimen shows the form of the mouth very neai-ly complete. 



7, Ammonites Bunburianus, Sharpe. Plate IX, fig. 3. 



A. testa discoided, compressd, lateribus transversim multi-sulcatd, dorso tri-seriatim 

 tuberculatd : umbilico parvo ; aperturd ovato-rhomboided. 



Shell discoidal with flattened sides ; crossed by twenty-five unequal furrows, continued 

 over the back, which is marked by three rows of tubercles ; the furrows are very small 

 near the umbilicus, and gradually grow wider and deeper towards the back where they 

 are most important : on the back the spaces between the furrows form broad ribs, each of 

 which has three longitudinally flattened tubercles, one on the middle and one on each side 



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