288 
INPEEIOR OOLITE AMMONITES. 
and especially with the genus Sonninia ; but when speaking of either new or old 
species I wish to make a certain reservation. The series of specimens is generally 
so extraordinarily complete, at any rate in Sonninia, that division into species is 
often purely arbitrary. The "species" are simply different gradations in deve- 
lopment ; and, owing to a generally profuse ornamentation, small differences are 
rendered fairly striking. My object will be to show the gradual development of 
the members of the different genera; and the word "species," or a so-called 
" specific name," will only be used as a means to attain that end, and as presenting 
after all the shortest method of nomenclature. 
The Origin of the SoNNiNiNiE. — As the Sonnininge are placed in the family 
Amaltheidge on account of genealogical reasons, their origin becomes a matter 
of some importance. In my paper on Sonninia, &c.,' I mentioned the strong 
resemblance between the inner whorls of Amaltlieus margaritatus (PL XLIX, 
figs. 1, 1 a), Pleuroceras"^ spinatum (PI. XLIX, fig. 7), and Sonninia 
suhspinosa (PI. XLIX, fig. 8) ; and I supposed that a keelless coronate form, 
like the figures in the plate, was the ancestor of all these genera. In the case 
of Amaltlieus and Pleuroceras I overlooked the sulcate-carinate form of Amah 
coronatus, to which Dr. Haug has since drawn my attention ; and it seems to me 
that, with a certain allowance for what may be termed " the modifying action of 
the law of earlier inheritance,"^ these two genera are derived from a common 
' ' Quart. Journ. G-eol. Soc' vol. xlv. 
2 Haug points out (' Annuaire geol. universel,' tome vi, p. 835, foot-note, 1890) that the name 
^^Pleuroceras" has been in use since 1849 for a genus of Gasteropoda. He further objects to the 
employment of two names, and would unite these Ammonites as Amaltheus. I do not find myself 
able to agree with this method, because in biological investigations it is certainly a help to have 
separate generic names for Ammonites which dilfer essentially in certain developmental features. 
While Amaltheus only reaches what may be called " the unispinous stage," and then declines to a 
costate stage (see foot-note nest page), the later branch, " Pleuroceras,''^ reaches a " multispinous 
stage " before it begins to decline. It also retains its furrows during retrogression, which Amaltlieus 
does not. I, therefore, use the term ''Pleuroceras" only until another be proposed. 
^ By this term I describe a phenomenon which has been overlooked in ontogenetical investigations. 
Hitherto it has been supposed that the inner whorls of an Ammonite represented exactly the 
developments of the earlier members of the phylogenetic series to which it belonged ; that, in fact, the 
inner whorls revealed the genealogy. Such, however, is not altogether the case. The embryonic and 
infantile whorls do not exhibit all the characters of the stage which they represent — in fact, earlier 
inheritance acts partially, in that it allows certain characters to be retained and not others. 
Presumably only those characters adapted to the requirements of the animal at that stage of life are 
retained, while others disappear. The young specimen of Amaltlieus nudus, Quenstedt {priestalilis, 
Hyatt), PI. XLIX, figs. 3, 3 a, shows this phenomenon. This species is the descendant oi Amah 
margaritatus (inner whorls depicted PI. XLIX, figs. 1, 2) ; but instead of showing in its youth a 
crenulate carina correlating with ribs as in the adolescent " margaritatus" it has a plain abdomen ; 
and the crenulate carina is not developed until much later in life than in " viargaritatus." Similar 
features are seen in " Plewoceras " pseudo-costatum (" Descent of Sonninia," 'Quart. Journ. GeoL 
