80 ME. H. WOODS ON THE MOLLUSCA [Feb. 1 896, 



than the siphonal saddle, inferior-lateral about two-thirds the size 

 of the superior-lateral saddle. 



In older individuals the whorls become more flattened, and the 

 tubercles and ribs (11 to 14 on the last whorl) less distinct ; the 

 latter are less curved and all start from the tubercles at the margin 

 of the umbilicus and often disappear on the siphonal area. In 

 senile forms the shell is quite smooth. 



Remarks. — The average diameter of this species in the Chalk 

 Bock is 1| inch ; Dr. Morison has a specimen 8 inches in diameter 

 from the Luton cutting. In the zones of the Middle Chalk below 

 the Chalk Bock it is often much larger, being sometimes more than 

 a foot in diameter. In one or two cases only have I seen portions 

 of the shell preserved. 



The specimens described by Mantell came from near Lewes and 

 Eastbourne; and, although he gave no figures, there can be no 

 doubt whatever about the identity of the species. Had there been 

 any, it would be removed by the fact that the specimen figured by 

 Sowerby a year later in the ' Mineral Conchology ' was presented 

 to him by Mantell himself. Sowerby gave only one figure, that of 

 a rather old individual 9 inches in diameter. D'Orbigny figured in 

 the l Paleontologie Francaise ' the adult and young forms under 

 the names of Ammonites peramplus and Ammonites Prosperianus 

 respectively ; at the same time, he stated that the latter might be a 

 young individual of the former, but not having been able to observe 

 a passage between the two, he would regard them as distinct species. 

 However, almost all later authors, except Dixon (' Geol. Sussex, 7 

 1850), have considered Prosperianus as a synonym of peramplus. 

 Good figures of this species, showing examples of different ages, are 

 given by Sharpe ; his specimens came from the Middle Chalk 

 (probably Chalk Eock) of Hertfordshire and Wiltshire. 



Specimens from the Trichinopoly Group of the north-west of 

 Anaupaudy referred to this species are described and figured by 

 Stoliczka ; but these differ from the European examples in the smaller 

 ribs forming a tubercle ' at the point where they are curved 

 forward.' Stoliczka remarks (p. 131), 'D'Orbigny's Am. peramplus 

 seems to differ very considerably from the original English figures, 

 while his Am. Prosperianus is identical with them.' This is 

 certainly an erroneous observation, for Sowerby's figure agrees very 

 closely with d'Orbigny's peramplus. Stoliczka also figures under 

 the name of Ammonites Vaju a specimen from the Trichinopoly 

 Group of the north-west of Anaupaudy, which, if not identical with 

 peramplus, is certainly its representative form. 



Ammonites Jlaccidicostata, described by Ebmer from Guadalupe in 

 Texas, is near to peramplus, but it has only one small rib between 

 the larger ones. 



Ammonites fraternus of Gabb, from the Martinez group of 

 Benicia, California, differs only in the form of the suture-line — the 

 siphonal saddle is more divided than in peramplus, and is larger 

 than the superior-lateral saddle. 



The form described by Eedtenbacher under the name of Ammonites 



