GRAMMOCERAS. 163 



conclusion that it is in reality not the earliest species of the genus. Siich a 

 species we should expect to find possessed of evolute quadrate whorls, coarse 

 ribs, and a sulcate ventral area. 



Until we reach the Jurense-zone the appearance of Grammoceras is little more 

 than fitful. Some of the better known species of the genus are Grammoceras 

 Normani(mu7n(d'0vhifi;vij) and Gramm.Kurrianum (Oppel) intheMa7'(jaritat7ts-zoTie, 

 Grammoceras acutum (Tate) in the Spinat/mn-zone, Gramm. ovatum (Young and 

 Bird), and Gramm. capillatum (Denckmann) in the Gomm,u7ie-zone. 



With the advent of the Jurense- and Opaliiium-zones, a large number of species 

 of Grammoceras are found ; in fact, the genus may be said to dominate that period 

 exclusively. At the end of the Opalinum-zoue it dies out abruptly, and only one 

 dwarf species is found in the MitrchlsoTise-zone. Some of the species which are 

 found in the Jurense-zone, such as Gramm.. toarcense, Gramm. quadratum, &c., are 

 less developed — have more resemblance to the supposed parent form — than 

 those that are met with earlier. This is a curious fact ; and we must either 

 suppose that these species have reverted, or else that the immediate ancestors of 

 some of the later forms have not yet come to light. I prefer this latter theory, 

 because we find no trace of any reversion in the young of the later forms, and we 

 • see that the species develop riormally during the Jurense- and Opa,linum-zones. 

 I do not therefore consider that Gramm. antiquum is the actual ancestor of Gramm. 

 toarcense, nor that — in spite of their resemblance in shape — Gramon. Kurrianum, 

 with its knife-like, carinate, ventral area, is the parent of the later Gramm. 

 fallaciosuyn with its well-marked ventral area and high hollow carina. 



The species of the genus with which I have to deal may be divided roughly 

 into two groups : (1) the Sf7iatulum,-grou]) — the species with a solid carina ; (2) the 

 Fallaciosu7n-gron'p — the species with a hollow carina. Whether the difierence in the 

 carina indicates a difi"erent origin for the members of the two groups I am not in a 

 position to say. In the horizon with which I have to deal at present the two groups 

 are certainly clearly defined ; but my difficulty consists in not having been able to 

 obtain specimens of the earlier species showing the structure of the carina. 



I have said that Grammioceras dominates the Jurense- and Opaliirmn-zone'& ; 

 and what is more, its geological range in this country is practically limited to the 

 district of the South Cotteswolds, which lies between Haresfield Hill on the north 

 and Sodbury on the south. I do not say that Grammoce7-as occurs nowhere else, 

 but that the above-named district may be considered its headquarters. I have 

 previously given several sections of this district (pp. 43, 44) ; but further research 

 has shown that the strata are capable of more minute division, and that their 

 correlation with the Dorset-Somerset strata — a point at that time in much 

 obscurity — is thereby more easily explained. In addition, further work in the 



