448 INFERIOR OOLITE AMMONITES. 



shell. III. Costate, in which the amount of tuberculatum is entirely subordinate. 

 IV. Sub costate, in which the degree of costation is decidedly feeble. V. Smooth, 

 in which the test is practically smooth, or costation takes the form of irregular wave- 

 like bulgings. VI. Renovate, in which are placed species that show in a marked 

 degree a tendency to increased ornamentation as they grow older. 



Of these six columns, the first three and No. V are subdivided, each into two 

 parts : I. Tuberculate, a and b for species more or less strongly tuberculate 

 respectively ; II. Tuberculate-costate, a for species which have a strong costate 

 stage succeeding the spinous stage ; b for those which have a weak costate stage ; 

 III. Costate, a for those in which the central whorls are obviously tuberculate ; 

 b, species in which the tuberculation is so rudimentary that it is generally 

 unobserved ; V into a, containing species with a concentric umbilicus, as 

 opposed to b, possessing species with an excentric umbilicus. By this means ten 

 columns are obtained, which divided into the number of species, 71, gives about 

 7 species to each column as an average. It will be noted, however, that there is 

 very considerable difference in the number of species in the columns : III a, carries 

 off the palm with 16 species, and III b has only 2 to boast of. 



Having found the column into which its external ornamentation places a 

 specimen to be identified, it is obvious that the next step would be the comparison 

 in turn with the figures of each of the species named in the column. In the case 

 of the columns with a small number of species this would not take long ; in the 

 case of a column like III a it would be advisable for the student to make a further 

 analysis. First he can separate the species with reclinate costse from those with 

 costse more or less upright, putting on the one side Sonn. cymatera, and on the 

 other side Sonn. dominatrix. Having settled which of these two groups his fossil 

 belongs to, he may proceed to divide again — those with a large umbilicus like 

 Sonn. euromphalica, those with a small umbilicus like Sonn. dominans. The number 

 of species to choose from would not then be large ; but it may be further reduced 

 by dividing according to relative intensity of costation, and lastly by the different 

 degrees of thickness. For illustration of this analytical method of work the 

 largest group has been taken ; but of course the same plan may be pursued with 

 the others, though it will not be so necessary. Further, in the above case only the 

 external ornament has been taken into consideration. If, however, the septa be 

 shown, their details should, from the analyses given in pp. 384 — 391, lead to 

 correct identification. Only a limited number of species will be found in the 

 Classification having a common type of suture-line, and of this number nearly 

 every one would be found to occupy a different column in the Table of Morphic 

 Equivalents on account of external difference. Therefore, if these two tables be 

 used in conjunction, the specimen, if it belong to one of the species figured, 

 should be identified with certainty. 



