l6  TYPE AMMONITES— VI Feb. 



1926 



unlike the prior virgatite or the pectinate respectively ; but as the 

 magnicostate character is sequent to the respective parvicostate stages, 

 it would not justify generic separation of magnicostates from parvi- 

 costates. 



KosMOCERATES. There has now been figured a sufficient number 

 of genera, which have hitherto come under the designation of Kosmo- 

 ceratidae, to make possible analyses of their characters in tabular form. 

 This is by far the most concise method of showing how genera differ 

 one from another, and what progression or regression they make in the 

 matter of development. This method is preferable to that of describing 

 single genera promiscuously ; but its disadvantage is that time must 

 elapse before a sufficient number of genera have been illustrated. 



The subsequent Tables III — V give analyses under the following 

 headings : — 



Shape of Venter — runcinate, or subsulcate, or sulcate : these are 

 the progressive stages — towards greater heterogeneity ; then follow 

 reverse stages — to homogeneity or less heterogeneity — subsulcate, 

 runcinate, rounded. 



Ornament of Venter — progressive — ribs, tubercles on the end of 

 single ribs, tubercles into which there flow 2, 3, 4 or possibly more 

 secondary ribs ; then the reverse, 4, 3, 2, i, to none, and finally to 

 smoothness — loss of ribs. 



Ornament of lateral area — progressive — costate, one median row 

 of tubercles, two rows of tubercles, one being median and the other 

 inner marginal (umbilical) ; then the reverse — back to one row of 

 tubercles after having had two rows, costate merely, after loss of all 

 tubercles, and a singular development in a limited series — back to 

 smoothness, but retaining and even enlarging the inner marginal tubercles: 

 this is a singular phenomenon, because in most cases the rule is ' the last 

 character to come is the first to go ' : this departs from that rule. 



There are other characters which might be used in analyses : strength 

 of primary ribs, number of secondary ribs to primary- — from one to 

 perhaps seven and back again ; and all the various details of suture- 

 line — relative length of EL, Li, L2, characters of, say, Li from the 

 simple trefoil or clover-leaf pattern to the long, elaborate, thin-stemmed 

 cruciform. All such characters have to be utilized in the naming and 

 distinguishing of genera, but their analysis is not regarded as necessary 

 now. 



However, in the analyses of the characters which are made it will 

 be obvious that there is often a difficulty in saying whether a given 

 character is in a pre- or a post-condition : for instance, a runcinate 

 venter is obviously post-round ; but is it pre-sulcate or post-sulcate ? 

 Have the members of a given genus, which show a runcinate venter, 

 been through a sulcate stage and returned to the runcinate ? Or is a 

 runcinate venter the highest development to which they reached, 

 so that the next move is back, to rounded again ? 



Ontogenetic studies may sometimes answer these questions ; but 

 not always — for there is ever the possibility of a stage being skipped. 

 Then runcinate pre-sulcate and runcinate post-sulcate merge one into 

 the other. 



