1874.] 231 [Hyatt. 



low interior of the keel. One of these, in the collection at Semur, 

 exhibited the hollow, but I found it filled with layers evidently of or- 

 ganic origin. The black layer was also present. This would appear 

 to be an intermediate stage between the solid and hollow keeled 

 groups of Ammonites, but it may be only a specific character. That 

 it is not due to age may be proved by the examination of young 

 specimens in which also the same solid filling may be observed. 



In 0. Guibalianum, Guibalii, and Lotharingum the hollow keel was 

 also observed. The late stage of growth at which it appears and the 

 comparatively early stage at which it disappears, as well as the com- 

 pleteness of its obsolescence, are very marked in 0. Guibalii and espe- 

 cially in 0. Lotharingum. The young of 0. oxynotum and 0. Guibalia- 

 num and Guibalii are very similar in many of their varieties to those of 

 Agassiceras striaries and in 0. oxynotum this is so marked that it becomes 

 almost identity. The young of 0. Lotharingum, however, never show 

 these striaries-like forms, but begin at a very early stage to resemble 

 the adult of 0. Guibalianum in all its characteristics. These facts 

 appear to justify the conclusion that we have in this family either the 

 first appearance, or at least one independent source, of the hollow- 

 keeled group springing up in a genus whose origin is traceable by 

 inference from the developmental characteristics to the Arietian spe- 

 cies Agassiceras striaries. The resemblances of the senile stage to its 

 own young in the form and characteristics of the last whorl are very 

 remarkable. The length of time during which the adult stage main- 

 tains the peculiar sharpness of the whorl and the keel in the indi- 

 vidual is greatest in 0. oxynotum, less in 0. Guibalianum, still less in 

 0. Guibalii, and least in 0. Lotharingum. 



This also would be the natural order of the arrangement of the 

 species if placed by the evidence afforded by their development and 

 adult characteristics. 0. Guibalianum is in every way most nearly 

 allied to 0. oxynotum. There are some varieties, especially among 

 the German forms in Prof. Fraas' collection, which have much 

 sharper abdomens than the true Guibalianum and approximate very 

 closely to the stouter varieties of oxynotum. 



On the other hand, the duration of the striaries-like stage in 0. 

 Guibalianum, the septa of the adult, which are simpler or more Arie- 

 tian in outline than in oxynotum, and the essentially hollow keel, seem 

 to indicate a separate though common origin from Agassiceras stria- 

 ries. The specimen 0. Guibalianum in the Stuttgart Museum, in which 

 the hollow keel was observed, had an outer shell remarkably thick- 



