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INFERIOR OOLITE AMMONITES. 



reclining, ventrally-mclined ribs occasionally bearing spines, later with ribs only. 

 Ventral area obtusely arched, carrying a fairly prominent hollow carina. Inner 

 margin flat, high, but conspicuous only in the costate stage. Inclusion about 

 one-third. Umbilicus with large spines. Suture-line with cruciform superior 

 lateral lobe. 



This is a noteworthy species, chiefly remarkable for the large size of its spines, 

 and for a certain resemblance to Am. Sowerbyi, Miller. 



When I began the study of Inferior Oolite Ammonites I found that it was 

 usual to dispose in a very summary fashion of the species now known by the 

 generic name Son,uinia — the large-spined as Am. Sowerbyi, the small-spined and 

 costate ns Am. r.uriahil is. Under the name Soiverhiji, too, various spinous species 

 of Hammafocfirns were ranged ; and in many of our Museums have I noted this 

 wonderfully extensive range of this trivial name. 



Though quite unable to agree with this method of nomenclature, yet I was 

 sufficiently influenced by it to group this and one or two similarly-ornamented 

 shells under the name Sowerbyi, especially as I had not seen any example of the 

 true species. 



Since then I have had the opportunity of studying the original Am. Sowerbyi 

 in the Bristol Museum, and older examples, by the kindness of Mr. B. Wilson, 

 F.G.S. I find that Sowerby's is a very faithful drawing, and that the present 

 species is very distinct, being more inflated, larger-spined, narrower-whorled, and 

 more umbilicate. Throughout life there are many differences of ornamentation, 1 

 and the adult forms of Sowerbyi and magnispinata differ considerably in this 

 respect. 



How important geologically is absolute palasontological correctness may be 

 learnt from the results of incorrectness. Because it was, and often is now, thought 

 to be unreasonable to insist on what, especially in figures, seem to be small 

 differences, magnispinata and other forms were considered "sufficiently near" 

 Sowerbyi to be regarded as the same species. As a consequence the Concavum- 

 zone of Bradford Abbas was of course regarded as " Soiverbyi-zone ;" it was 

 described by that name ; and it retained that name for some years, as the early 

 pages of this Monograph show. Although this has now been corrected, yet there 

 can be little doubt that it will often be a cause of confusion and of difficulty. 



The size of the spines places this species near to acantlbodes and crassispinata, 

 but the earlier age at which the costate stage commences effectually separates it. 



becomes elliptical with age, and later still is almost oblong. The special meaning attached to the 

 word " to" used in the above and in subsequent diagnoses should be noted. 



1 In magnispinata there are two or three coarse costa? proceeding from the spine on the outer 

 area, in Sowerbyi four or five fine ribs. In the inner whorls the spines of magnispinata are close, 

 regular, and not separated by ribs ; in Sowerbyi they are distant, irregular, and separated by ribs. 



