Hyatt.] 6 [January 17 © ; 
The quadragonal form of the whorl and the tuberculated pile occa- 
sion in the young Deroceras minatum, during the later stages of 
erowth, a resemblance to Holandrez, which subsequently changes to 
a nearer resemblance to annulatum. 
Another series begins with Deroceras muticum of the Lower Lias, 
which appears in the Raricostatusbed, just above its nearest ally, 
Deroceras Dudressieri. :This species is very much smaller than the 
last named, and differs in other respects; but it resembles it more 
closely than Deroceras armatum; whether it compares in the same 
manner with Deroceras densinodum or: not, 1 am unable to decide. 
Its adult characteristics, however, and development, while they 
approximate to Deroceras armatum, do not permit of its association 
in the same series. The absence of intermediate pile, the con- 
stancy and closeness of the tuberculated pile and the septa are very 
distinct in this species, and the huge Deroceras nodogigas of the Mid- 
dle Lias, which appears to form the second term in the same genetic 
series. Both of these series exhibit, so far as J can trace, no rever- 
sionary characteristics. Whether the modifications may be consid- 
ered progressive or not, remains to be determined. There is the 
same tendency as exists in the other series of the family previously 
described to suppress the fold-like pile and prominent tubercles. It 
is noticeable that in the armatus series this takes the same direction 
as in the genetic series of the Dactyloide, which begins with Dacty- 
lioceras commune and ends with Dactylioceras Braunianum. The 
tendency in both of these groups is to obliterate the large tubercu- 
lated fold-like pile, with the minor ridges or pile gathered and join- 
ing in fascicles at the tubercles, and to substitute for this and the 
flattened abdomen a rounded whorl, single, sharply defined, entire 
pile. It must be remembered, also, that in neither of these series is 
there any increase of the involution in successive species. The 
umbilici are entirely open. The septa of Deroceras Davi are rather 
more ornate and complicated than those of Deroceras armatum, and 
the lobes and cells of the latter than those of Deroceras Dudressieri. 
A certain amount of progress may perhaps be claimed for this series, 
but the other series, that of Deroceras muticum, is too incomplete. De- 
roceras muticum is certainly much smaller than its immediate ancestor, 
Deroceras Dudressieri, but on the other hand, Deroceras nodogigas is 
just as large. The septa, also, of the first named, which can hardly 
be said to be more complicated, cannot be claimed as simpler than 
those of Deroceras Dudressieri. ‘The varieties of Deroceras muticum 
