178 CRETACEOUS LAMELLIBRANCHIA. 
rather irregularly developed, but in others they are placed at yery regular intervals, 
and are of very uniform size. The differences in the spies 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 im 
the ornamentation of this species. 
A few large specimens (length 67 mm., height 72 mm ), from the Bb. mucronata 
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 mterspaces; these forms agree perfectly im 
outline and in the form of the ears with some of the large examples of P. 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. 2189), or, 
in other cases, just below well-marked growth-lines near the ventral border 
of the valve, where the shell has undergone less wearing (Norwich Museum, 
No. 2188). 
In this variable species the extremes differ to such an extent im their orna- 
mentation that, when 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, moreoyer, 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 (¢. g. Hast 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. 1’. 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 ribs (found at Trimingham, ete.) agrees with the 
example figured as 1’. undulatus by Holzapfel. Another form with numerous 
well-developed, regularly placed, spiny scales on the fairly numerous radial ribs 
approaches P. serratus. 
It is very difficult to decide which of the two names, crelosus 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 Ist, 1822, 
and the work was received by the Geological Society before the end of June of 
that year. Mr. C. D. Sherborn informs me that the work of Cuvier was noticed as 
' T have had the opportunity of studying over 200 examples. 
* Dr. Blackmore, who has made a very large collection of P. cretosus, has independently come to 
the same conclusion—that all are forms of one species. 
