EXTIITCTION OF AN^IMAL GROUPS. 301 



the typical Cretaceous area of the peninsula of India as representing 

 a series of nearly continuous depositions from the Neocomian to the 

 Danian, inclusive, we find that there has been a very sudden dimi- 

 nution in the number of species before the top of the series is 

 reached. In the Lower Cretaceous division, the Utatur group, cor- 

 responding to the Cenomanian of continental geologists, the num- 

 ber of species of coiled Ammonites is, according to Stoliczka, 67 ; 

 in the middle division, Triohinopoly group (Turonian), 30; and in 

 the upper division, Arialur group (Sennonian), 21. lu the upper- 

 most Arialur beds of Ninnyur, which probably correspond to the 

 " Maestrichtian, " or Danian, not a single species is found. The 

 seeming anomaly that the upper and middle divisions of the Cre- 

 taceous (Arialur and Trichinopoly groups) should contain an equiva- 

 lent numljer of species is not exactly in harmony with the law of 

 gradual numeric diminution ; but its explanation is, doubtless, found 

 in the fact that the development of the Arialur deposits is double 

 that of the underlying deposits of the Trichinopoly group. °° In the 

 typical Cretaceous areas of England there is an equally well-marked 

 reduction in the number of species before the end of the series is 

 reached, and the same, although to a less extent, is also the case in 

 France. In California, where the breaks in the Cretaceous series 

 appear to have been of comparatively insignificant value, and where 

 a nearly continuous sedimentation tides over the gap which else- 

 where exists between the Cretaceous and the Tertiary, the disap- 

 pearance of the ammouitic fauna is a very gradual one. In the 

 lowest member of the system, the Shasta group, the coiled forms 

 number ten species ; in the Chico group, six ; and in the Martifiez 

 group, the top of the system, two. But, to draw out the line still 

 further, we have here the indisputable passage of one species into 

 the dsposits of the Tejon group, the base of the Tertiary series 

 (Eocene). This unique case of an ammonite surviving the Me- 

 sozoic fauna, which will be, doubtless, repeated at many parts of 

 the earth's surface not yet explored by the geologist, finds its paral- 

 lel in the similar survival in Australia of a solitary belemnite, if 

 the organism so described really proves to be such. Less grad- 

 ual in their disappearance, apparently, than the true ammonites, 

 are the uncoiled forms of the same family, whose remarkable de- 

 velopment, just before the close of the Mesozoic era, must be 

 considered one of the most striking facts in paleontology. But 



