178 CRETACEOUS LAMELLIBRANCHIA. 



rather irregularly developed, but in others they are placed at very regular intervals, 

 and are of very uniform size. The differences in the spines are in some cases 

 connected with the state of preservation of the specimens. Nothing less than a 

 large series of micro-photographs would illustrate adequately all the differences in 

 the ornamentation of this species. 



A few large specimens (length G7 nun., height 72 mm ), from the B. mvcronata 

 zone of Norwich, appear at first sight to differ greatly from this species, on 

 account of the greater part of the valves being nearly smooth, or marked with 

 linear radial grooves separating flat interspaces ; these forms agree perfectly in 

 outline and in the form of the ears with some of the large examples of 1\ cretosus, 

 and I believe they are only large and much-worn examples of that species, since 

 the characteristic ornamentation sometimes occurs in the neighbourhood of 

 the umbo for a distance of about 10 mm. (Norwich Museum, No. 2180), or, 

 in other cases, just below Avell-marked groAvth-lines near the ventral liorder 

 of the valve, where the shell has undergone less wearing (Norwich Museum, 

 No. 2188). 



In tliis variable species the extremes differ to such an extent in their orna- 

 mentation that, Avhen considered separately, they appear as distinct species, but 

 on examining a large series of specimens ^ a complete gradation is found to exist 

 between the different forms, which we must therefore regard as merely modifications 

 of one species.^ The different forms, moreover, are not confined to one horizon or 

 one locality, and it seems therefore hardly necessary to give them distinct names ; 

 but from the same zone and the same spot (e. g. East Harnham and Norwich) extreme 

 forms and connecting links may sometimes be found. Moreover, in some cases two 

 types of ornamentation appear on different parts of the same shell. The form with 

 numerous strong ribs agrees with the type of P. cretosus, Defrance. /'. arachnoides, of 

 the same author, is the variety with relatively few and narrow ribs, with the spines 

 poorly developed, and with regularly-placed concentric ridges. A variety with 

 very slender and numerous I'ibs (found at Trimingham, etc.) agrees with the 

 example figured as 1'. iindnlafns by Holzapfel. Another form with numerous 

 well-developed, regularly placed, spiny scales on the fairly numerous radial ribs 

 approaches F. senatus. 



It is very difficult to decide which of the two names, creto.sus or nitidus, has the 

 priority, since the exact dates of publication of the works of Cuvier and Mantell 

 cannot be determined at present. Mantell's preface is dated May 1st, 1822, 

 and the work was received l)y the Geological Society before the end of June of 

 that year. Mr. C. I). Sherljorn informs me that the work of Cuvier was noticed as 



' I have had the o]jport.iiuity of stud_)iuy over 200 oxani[)les. 



- Dr. Blackmore, who has made a very large collectiou of P. crdosm, has iiidi'iieiidently come to 

 the same conclusion — that all are forms of one species. 



