11 



INFERIOR OOLITE AMMONITES. 



is found to give a criterion with so much diversity of character, it may well, until 

 knowledge of the family be more complete, be utilised, with the suture-line, as a 

 basis for a classification. 



By the term ornament it is not intended to imply merely the production of 

 costa? and tubercles. They practically indicate stages of phyletic development, 

 and no more. It is not the kind, but the manner of ornament which has to be 

 considered, namely, the disposition of the growth-lines, for in these cases the 

 ornament is parallel therewith. The disposition of the ornament, then, is found 

 to have very remarkable variation in the Hildoceratidce ; and, as this disposition 

 is the same as the growth-lines, which were the results of the two processes of 

 deposition and absorption on the part of the mantle during the growth of the 

 specimen, it must indicate certain anatomical differences on the part of the various 

 species. 



Therefore in the Hildoceratidse the disposition of the ornament, or, what is the 

 same thing, of the growth-lines, is considered to be of particular importance. At 

 any rate, it is regarded as a test of generic affinity. This disposition of the 

 ornament is used in illustration, and referred to as the radial line or curve. 1 



Such are the reasons for, and the methods of the revision. No one regrets more 

 than the author of this work that the revision will mean the alteration of many 

 names, involving the disturbance of what appeared to be settled. But it cannot 

 be helped. It may be said with Cicero, Errorem creat similitudo ; and what may 

 be called the deceptions of homoeomorphy are only being gradually learnt. I con- 

 fess that they have misled me. It is no excuse if I have been deceived in good 

 company. But I recognise some of the incorrectness of my own work. I would 

 attempt to alter this, and to place the whole on a surer basis, hopeful that I am 

 wiser to-day than I was yesterday. 



In order to carry out the revision with conciseness, and to deal with an 

 immense mass of detail, it becomes very necessary to introduce certain definite 

 technical terms. Some of them have been already employed in earlier portions of 

 this work ; but the following notes are intended to save the labour of reference in 

 regard to the old, and to explain the meanings of the new terms. 



Shape of Whorls. — Concise terms to express the differences of whorl-shape are 

 required. Some of the following terms are already used in connection with 

 Gastropoda, and so I have extended them to make a congruous series. 



An Ammonite may reach a given diameter by making several narrow, or a 



few broad whorls. In the first case, in reference to the number of whorls, it 



would be polygyral ; in the second, oligogyral ; while in reference to the breadth 



of the whorls — from inner margin to periphery — it would be stenogyral, narrow- 



1 The radial curve and the suture-liue both demand consideration. In certain cases there is an 

 identical form of radial curve, but a marked difference in suture-line. There are some striking cases 

 of this deceptive hoinceoinorphy. 



