12 TYPE AMMONITES— III Mar. 



accuracy, care being taken in reading the lower numbers, and it can 

 be used for any size of specimen. When dimensions have been given 

 instead of proportions, as is the case in many works, they can be 

 translated by simple reading of the rule. 



When photographs are measured instead of specimens the results 

 may be slightly at variance — chiefly from the camera giving a widening 

 of the umbilicus at the expense of the whorl. Measurements from 

 photographs may, therefore, advisedly be marked <f> 



Systematic 



As an introduction to this subject it is necessary to make some 

 remarks with a view to providing certain short cuts in descriptive 

 matter. 



Abbreviations: Photograph, </>, see above; other abbreviations, 

 see following paragraphs. 



Tuberculation : For distinguishing Ammonite genera particular 

 attention requires to be paid to the position of the tubercle on the side 

 of the whorl. Thus a distinction between Trilobiticeras and Otoites 

 is that in the former the tubercle is in the line of the lateral auricle — 

 actually in certain cases, a rare phenomenon, (T. trilobitoides PI. CXL) 

 it is found on the auricle itself ; but in Otoites the tubercle is in the line 

 of the inside of the auricle. Then in the so-called zigzag series (or one 

 section of it, Procerites) what is analagous to, and possibly a develop- 

 ment of the tubercle, the parabolic curve, is found in the line of the 

 outside of the auricle (PL CLIII). It becomes necessary to express 

 these phenomena with brevity. Reading from left to right, the specimen 

 is placed with its periphery to the left, and therefore in Procerites the 

 tubercle (or its analogue) is O Lap. — before the lappet ; but in Trilo- 

 biticeras it is (Lap.), this does not mean that it is on the lappet itself, 

 but that it is in the line of the lappet. However, when the lappet is 

 broad enough further particularization may be necessary. The tubercle 

 may be in the line of the outside, of the middle, or the inside portion, 

 which can with brevity be expressed as (Lap. Z, 2 or 3) respectively. 

 Trilobiticeras is (Lap. 2), Epalxites is (Lap. 3). 



Useful, however, as observation of position with regard to the lappet 

 may be, yet that is not a constant feature. The suture-line therefore 

 can be taken as a guide, alone or in addition. The varying position of 

 the tubercle with regard to the suture-line has long been recognized 

 as a generic character, though it has been insufficiently employed : 

 its different locations on the lateral area — a less satisfactory guide — 

 have more usually been considered. Now Dr. A. E. Trueman, in an 

 excellent study (Evol. Lip. ; Q.J.G.S. LXXIV, 1919, p. 247) has 

 graphically illustrated (p. 258) the different positions of tubercle to 

 lobes of suture-line in a detailed series of genera. 



For reading the suture-line the specimen is placed in the same 

 position, venter to left, inner part to right, that is, EL, L 1 , L 2 and so on. 

 Position with regard, say, to L 1 may be O L 1 , that is, before L l is 

 reached, (L 1 ) — that is in the line of the area of L x , — and L l O, that is 

 after L 1 . When lobes are wide-stemmed further precision may be 

 attained as with the lappet — (L 1 1, 2 or 3). The saddles can be used 

 in the same way. 



