SUPPLEMENT.— TERMINOLOGY. 



in 



whorled, or platygyral, broad- whorled. Then there is the other dimension of the 

 whorl to be considered — from side to side. If the Ammonite has thick whorls it 

 is pachygyral ; if thin compressed whorls it is leptogyral. 



Radius. — This is the comprehensive term for the ornament which is parallel 

 with the growth-lines. It gives no qualification as to its direction, size, shape, &c. 



Direction of Radius. — This has to be considered under two headings : 



1. The radius is straight or curved on the lateral area. In the first place it 

 is a rectiradius, and the specimen is rectiradiate ; in the second, a flexiradius. 



2. The recti- or flexiradius has a general direction across the whorl in regard 

 to a straight line drawn from the centre of the Ammonite through the inner end 

 of the radius : (a) it either falls behind that line on its way to the periphery, — 

 it is then a rursiradius, and the specimen is rursiradiate ; (b) it has, in its general 

 direction, the same course as the line, — it is then a versiradius ; (c) it tends 

 forwards of the line, — it is then a prorsiradius, and the specimen is prorsiradiate. 1 

 When the radius is so disposed as to be a combination of prorsi- and rursiradius, 

 forming a >, the apex pointing towards the aperture of the conch, it may be 

 called an anguliradius, and the specimen would be angnliradiate. 



It is convenient to have similar terms in connection with the manner in which 

 the radius crosses the periphery. As the radial curve is drawn in comparison 

 with a straight line from the centre to the position attained by the radius on the 

 medial line of the periphery, it makes much difference whether the specimen be 

 peripherally anguliradiate or rectiradiate. 



Stria. — This describes the radius when it is like any slightly raised line. 



Costa. — This signifies a coarser ornamentation in the form of a ridge. Accord- 

 ing to the size of its costse a specimen may be parvicostate or crassicostate ; 

 according to the number, paucicostate or densicostate. 



When the radius is part costa and part stria the terms may be strii-costa or 

 costi-stria, commencing from the inner margin. 



Tuberculation. — A raising of the test, more restricted than a costa, forms a 

 tubercle — as a general term. A conical elevation is a spina; an obtuse elevation, 

 more or less laterally compressed, a bulla; 2 a round knob, a nodus; a small 

 pimple-like elevation, a papilla. 



In certain cases the tubercle is partitioned off by a layer of test at the base. 3 

 In other cases it is not, and the interior must have been occupied by the 



1 Bursum, backwards; versum, turned in the direction of; prorsvm, forwards. 



2 In the cases under consideration the long axis of the bulla is across the whorl parallel with the 

 radius. To meet other cases further nomenclature will be required. It is obvious that these terms 

 are not sufficient. 



3 In Deroceras (Am. arm at us group) : when the test is lost there are no tubercles on the core, 

 but only plate-like areas. 



