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INFERIOR OOLITE AMMONITES. 



divided by a small but well-marked, hollow carina. Inner margin well-marked, 

 upright, flat. Inclusion about three-eighths. Suture-line with a somewhat 

 equally -balanced superior lateral lobe. 



This species is easily separable from semispinata by its shorter spinous stage, 

 less marked costas, more compressed form, and more elevated carina. It is, 

 however, in general appearance exactly a reduced edition of semispinata except 

 in one feature — the carina is more elevated. It has, however, just the same style 

 of spinous stage, and just the same reclining ribs, becoming more upright and 

 obscure with age. It has the same flat-sided whorl and marked inner margin, 

 so that, in spite of its keel, I feel compelled to consider it a dwarf development 

 of semispinata. 



From spinifera or cymatera it is easily distinguished by its flat whorls, distinct 

 inner margin, smaller umbilicus, &c. From its morphological equivalents in the 

 dominans- or submarginata- stocks it is separable by its reclining ribs, which give a 

 twisted appearance to the umbilicus. This is presumably an adult form, for 

 I have from the Bradford- Abbas beds sundry examples, about this size, agreeing 

 in general character with the specimen which I have selected as the type, 

 figured in PI. LXXV, figs. 6—8. 



The biplicata-s toe k . 



The forms which comprise this series are easily separable from those hitherto 

 described by a peculiar feature of the costate stage, namely, that the ribs are 

 arranged in irregular pairs, separated by one or more somewhat obscure ribs ; and 

 then, later on, there tends to be an alternation of obscure and prominent single 

 costse. 



There are four forms which exhibit this peculiar style of ornamentation. They 

 are easily separable one from another, and seem to belong to two slightly distinct 

 genetic series ; yet I shall for the present describe them under only two names, 

 thus : 



Ribs (or growth-lines) ventrally inclined : 



S. biplicata a. For the most part costate with the ribs in pairs ; later, ribs 

 alternately prominent and obscure. 



8. alternata «. Ribs less prominent; the "pairing" less marked, the 

 " alternation " well shown. Whorls flatter-sided and broader. 



