MOEPHOLOGT OF ' STEPHANOCEEAS ' ZIGZAG. 449 



whorls — and it has lost the small knobs on the ribs. If present at 

 all, the zigzag-Tibhing must end very soon ; but the centre is im- 

 perfect in the specimen as well as in some older examples of the 

 same species. 



The characters of adult procerum are observable in the adolescent 

 stage of the next development (PI. XIII. figs. 3, 4), but there is a 

 very noticeable increase in the inclusion which has brought about a 

 renewal of the compression of the whorl. This form is very distinct 

 from the last on account of its small umbilicus, and may be known 

 as subprocerum. Still further increase of the inclusion produces 

 the form shown in PI. XIII. figs. 5, 6, in which, at the same 

 diameter, the umbilicus is considerably smaller. In the early adult 

 stage of this form it may be noticed that the primary ribs tend to 

 become obsolete and are much more distant — serving four or five 

 secondary ribs ^ — while the point of junction is altogether obscure. 

 There is also at this stage a tendency to widen the umbilicus again ; 

 the same tendency is shown in regard to the continuation of the 

 whorl in subprocerum. The form under discussion difiers from suh- 

 procerum in the various respects noted above, and it may be known 

 as clausiprocerum. 



Series j3. 



The thick form of zigzag, as the specimens are usually called, 

 is in reality either the parent of zigzag proper or else a less- 

 developed type of the stock from which zigzag came. It differs in 

 retaining the zigzag-i^h'hmg and the broad-abdomened whorls until 

 a later period of growth. For the sake of distinction it may be 

 called crassizigzag. The most pronounced of the series is a form 

 which retains its broad flat abdomen, to a diameter of 45 mm., a 

 long time in proportion to the others ; it may be called crassi- 

 zigzag a. 



Certain large examples, 130 to 155 mm. in diameter, though they 

 differ somewhat among themselves, may be regarded as adults of this 

 form, but, strictly speaking, they exhibit, each in their way, a certain 

 progress in development and are biologically later than the form in 

 question. One of them, in which the primary ribs have, so to 

 speak, run together to form large bulgings on the inner area, is a 

 most remarkable development. It is depicted in PL XIY. figs. 2, 3. 



The usual form of crassizigzag is shown in PI. XIII. figs. 7, 8, 

 and may be known as crassizigzag /3. A specimen which possesses the 

 complete mouth-border shows that it was without any lateral 

 lappets ; but perhaps this ought not to be considered as a distinc- 

 tion from zigzag. A larger specimen of the same subseries forms 

 the connexion with another example (PL XIV. figs. 4, 5) in which 

 there is a very distinct change, with a marked increase in inclusion 

 and an infiation of the whorl — in fact in every way a form homo- 

 plastic with the procerum-^Qiies. It differs, however, by its greater 



^ This is to a certain extent a reversion to the zigzag-rHbhixxg — so far as 

 concerns the number of secondary in relation to primary ribs. 



