DUxMORTIERIA MOOREI. 
257 
but acknowledged that Lycett's work was not accessible (' Monogr. Harpoceras,'' 
p. 671). 
It will be seen from the above remarks that Grammoceras mactra and Dum, 
Moorei were considered to be one and the same species ; and I myself was of this 
opinion when writing the earlier part of this Monograph. Hang's remarks upon 
the convergence of other species of Diimortieria and Grammoceras (" Polymor- 
pMdoi,'" p. 157, et seq.) first led me to investigate the relationship of these two 
species ; and it was, by following out Haug's observations to their logical 
I conclusions, that I ascertained the existence of the remarkable convergence 
between Gramm. mactra and Dimi. Moorei. 
Here it is that the extraordinary convergence between Grammoceras and 
Dumortieria reaches its culminating point. So peculiarly are these two species 
alike, that, had anyone to consider them alone, it would be very difficult to say what 
are the specific difi'erences. It is only a knowledge of their genealogy which 
shows why certain little-noticeable points of ribbing and suture-line are of such 
importance. (For the differences between the two species consult the generic 
definitions, and also p. 162.)^ 
The coarse distant ribs discernible in the inner whorls of this species are the 
only evidence remaining of its descent from Diim. Levesquei. The succeeding 
close-set ribs (or, really, coarse stri^) may be regarded as the " radians "-stage ; 
and there can be little doubt that Dum. Moorei is only the further development of 
Dum. radians by way of the specimen depicted PL XLII, figs. 6, 7/ Its finer 
ribbing and more compressed form are the chief features in which it differs from 
Dim. radians. 
The specimens depicted in my PL XLIY are really supplementary to Dr. 
Wright's figures. If the example he figured be regarded as the type, then the 
specimen shown in my figs. 4, 5 (PL XLIV), which is from Stoford, Somerset, 
^ Compared with Gramm. mactra, Bum. Moorei should show ribs straighter laterally, and less 
projected ventrally, longer lobes, and a more dependent iQner portion of suture-line. 
2 Since the above remarks were written (1890) the collection of more specimens seems to suggest 
that under the name Dum. Moorei are combined what are really further developments of various 
species — parallel developments in which fine striae are acquired very early in life. By the constantly 
earlier acquirements of these fine strise — a decadence of ribbing — Bum. radians produces one form ; 
by a similar decadence Bvmi. exigua evolves a similarly-ornamented, but wide-centred form ; and in 
like manner from Bum. radiosa arises a slightly thicker form ; these forms all acquiring the fine strise 
(formerly an adult character) at so early an age that what were at one time the distinctive specific 
characters have become obliterated. 
A similar state of things seems to obtain among the specimens referred to Gramm. mactra. There 
appears to be evidence to show that Gramm. aalense, Gramm. costulatum, Gramm. striatulum, &c., 
would all, by the "early inheritance" of fine strise, develop into what are now called " Gramm. 
mactra." Sach specimens may therefore be polygenetic, and in that case cannot properly be grouped 
1 under one specific name. 
33 
