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INFERIOR OOLITE AMMONITES. 



retrogression was not a continuous decline ; it was at times interrupted by what 

 were evidently progressive periods, especially among the species of the Concavum- 

 zone. These progressive periods appear more or less in the different genetic 

 series, so that there are some curious homoplastic results ; and there are in the 

 different series distinct evidences of alternate progressive and retrogressive 

 development. 



In order to understand the differences which separate the various species, it 

 is necessary to bear in mind that in retrogression all the various branches of 

 Sonninia change — 



1. From a spinous to a costate stage, and then to a smooth stage (irregu- 

 larity of the spines indicates decline of the spinous stage). 



2. From reclining to upright costse. 



3. From evolute to more and more involute whorls, or rather the coiling 

 becomes quicker. 



4. From thick to more and more compressed whorls. 



If these changes always occurred in unison there would be only one genetic 

 series, but such is not the case. In one series changes 1 and 2 make greater 

 relative progress than 3 and 4 ; in another the opposite prevails, and so on. The 

 differences between the various species, or rather morphological equivalents, of the 

 different branches depend on the different proportionate development of these 

 features in relation to each other. 



The Sonninise may be roughly divided into two main groups : 



1. The species of the Concavum-zone. 



2. The species of the higher horizons. 



The former are relatively the thicker in proportion to development when 

 morphological equivalents of the two groups are compared. The genus is 

 dominant in the Concavum- and so-called Sowerbyi-zones, is sparingly found in 

 the Sauzei-zone, and is unknown in the Hnmphriesianwn-zone proper. 1 The 

 majority of the species are of large size. As several species of the companion 

 genus in the Concavum-zone — Hammatoceras — are also known in the strata of Cap 

 San Vigilio, it is curious to reflect that no Sonninia has been described from 

 that locality, or in fact from any strata below the Concavum-zone. It is remark- 

 able that the questions where are the progressive forms, and whence did these 

 species come, must remain unanswered for the present ; so that on these points 



1 The highest horizon in which Sonninice are found is the Hump hriesianum- Sauzei-zone of 

 Dundry. The following alterations, which the reader is requested to make in p. 293, foot-note, were 

 accidentally omitted from the proof-sheets : — For Sauzei-zone, line 1, read Humpltriesianum- Sauzei- 

 zone. Opposite 7 place Sauzei- So werbyi- zone. In line 10, for " almost entirely absent " read " very 

 sparingly represented." Just sufficient species have been found in bed 7 to show that it contains in 

 part a slightly lower horizon than the Sauzei-zone of Oborne. 



