18S8.] 
19 
[Hyatt. 
practicable with classifications founded upon the principles usually 
accepted. 
The Arietidse comprise the following series : the first branch 
consists of Schlotheimia or the true Schl. angulatus group, and 
Wsehneroceras, a new genus, of which Week. ( Aegoc .) Panzneri (sp. 
Wdhner) is a good example. This genus is intermediate between 
Schlotheimia and Psiloceras, and is distinguishable by its young, 
which until a late stage of growth have whorls, with pilae and ab- 
domens like those of Psiloceras ; the pilae cross the abdomen, and 
the deep, well-defined channel of Schlotheimia is not developed at 
any stage. 
Psiloceras in a graphic presentation of the group occupies a cen- 
tral position ; this genus being the Radical Stock of the family 
and a direct descendant of more ancient forms. This genus bears 
the marks of this descent in its sutures, form and markings, since 
it is evidently a close ally of Gymnites of the Trias, and a modi- 
fied survivor of the trunk-stock of the genealogic tree of the order, 
which has bridged the gap between the Triassic and the Jurassic. 
The second branch of the radiating series which sprang from the 
focus of ancestral affinities centring in Psiloceras includes Caloce- 
ras, of which C. Johnstoni and C. raricostatum are good examples, 
and the genus Vermiceras, of which V. Conybeari is the typical 
species. 
A third branch begins with Arnioceras of which A. semicostatum 
is a good example, and from this, we can pass into Coroniceras of 
which O. Bucklandi is a typical form. A third branch begins with 
Agassiceras in which are included the typical forms afterwards de- 
scribed by Neumayer as Cymbites. A. Icevigatum is a well-known 
example of this series and in it is also included A. Scipionianum. 
The genus Asteroceras, of which A. obtusum is a typical example, 
belongs also to this branch and Oxynoticeras, of which 0 . oxyno- 
tum is the type, forms another branch. 
The branches may be traced to an origin in certain species more 
or less closely allied to Psiloceras caliphyllum. The Schlotheimian 
branch to Psil. planorbe var. plicatum; the Vermiceran branch, 
to C. Johnstoni closely allied to the same form, the Coroniceran 
branch can be brought to a focus in A. semicostatum and misera - 
bile , which have varieties frequently identified as Psil. planorbe var. 
levis by collectors ; the Asteroceran and Oxynoticeran branches 
can be followed with somewhat less certainty, but still w,ith very 
