part 4] THJE EYOLUTION OF LIASSTC GASTEOPODS. 



313 



are the most prominent throughout, with the consequence that 

 the whorl remains fairly flat, 



ProcnHtliium q^uinqiiegranosum is suflicientlj' similar to 

 P. ogerieni to he regarded as a member of the same stock ; it is 

 an advance on the latter, as shown by the increased number of 

 spirals and the greater curve of the axial ribs. 



A number of other specimens were examined from among* 

 material collected by one of us at Old Dalb}'^,'^ and a form which 

 ma}?" be regarded as intermediate between P. ogerieni 2iW^ quinque- 

 granosumwRs discovered. 



Observations made have been summarized in the following 

 table, in which P. ogerieni and P. quinquegranosum have been 

 inserted for comparison. 





Number of 

 smooth 

 whorls. 



Number of 

 spirals of 

 last whorl. 



First 



axials 



to appear. 



1 

 Number of Character 

 axials to a of axial 

 whorl. 1 ornamuit. 



A.. P. oqerieni 



H 



3 



5th or 6th 

 whorl. 



13 



Straight. 





B. Intermediate form ... 



n 



4 5th whorl. 



1 



14 



Curved. 



C. P. quinqnegranosum. 



1 



5 



4th whorl. 



16-18 



Sharply 

 curved. 



It will be noticed that specimen A in the above table has very 

 simple ornament, the first four whorls are smooth or ornamented 

 by spirals only, and the last whorl has only three rows of fairly 

 widely-spaced nodules ; while specimen C has simple ornament for 

 about three whorls only (axials appear by the fourth whorl), and by 

 the last whorl the ornament is accelerated to such an extent that 

 there are at least five rows of nodules present. It will also be 

 noticed that, Avhereas in A the axial ornament is straight, with 

 fewer ribs to the whorl, in C it is much more sharply curved, and 

 the ribs are finer and more closely packed. 



Proceeithtum cf. 8LATTEEI (Tate). (Text-fig. 4 c, p. 304.) 



Procerithium cf. slatteri Tate (20, p. 406, & pi. xxvi, fig. 7). 



Dimensions of s]oecimens figured here. 



Length. Breadth. Length of spire, 



b'b mm. 36 per cent. 67 per cent. 



Very small forms, with rounded or subquadrate whorls, orna- 



^ Now in the Geological Department, University College, Nottingham. 

 Typical specimens have been presented to the British Museum of Natural 

 History (G 24875-81). 



Spiral 



Sutural 



angle. 



angle. 



22° 



112° 



