GASTROPODA. 845 
Bellerophontacea.] 
to believe that the Heteropoda have been derived from the pleurotomarian line of 
development. In other words the Bellerophontacea, Emarginulide, Pleurotomariide 
and Heteropoda, though distinguishable, are nevertheless so closely united, in one way 
or another, that no system of classification can afford to separate them very widely. 
The most natural arrangement at present suggested is to let them follow that 
central type of the Gastropoda, the Patellacea, in the order named. This arrange- 
ment, however, must be regarded as only provisional, since it does not take into 
account the grand divisional line which ought to be drawn between the symmetric 
and asymmetric Gastropoda, according to which the class may be divided into two 
great groups, with the Scaphopoda, Pteropoda, Opisthobranchiata, Polyplacophora and 
Docoglossa on one side, and the Prosobranchiata, Heteropoda and Pulmonata on the 
other. These two groups are distinct already in the oldest known fossiliferous 
rocks—so far as known then, we may say from the beginning. The members of 
each are determined not so much by the presence or absence of strictly bilateral 
symmetry of organization as by their developmental history.* A tendency to 
become twisted characterizes the whole class, but while it is a constant condition in 
the second group, it is never as marked and often quite absent in the first. 
As regards the relation of the Bellerophontacea to the Pleurotomariide and 
so-called Zygobranchia in general, it 1s scarcely as intimate as usually supposed. So 
far as known both groups are equally ancient, and comparisons between the earliest 
types of each show that they are quite as distinct as any of the later ones. The 
apertural slit and slit-band which the two groups possess in common, and which 
is the only important feature in which they agree, became established somewhat 
earlier among the pleurotomarians than with the bellerophontids, from which it 
follows that the former is probably the more ancient instead of the younger type of 
the two. They may have had a common ancestor, but neither sprang from the 
other. 
The derivation of the bellerophontids is still obscure. Of all the known and 
sufficiently ancient types only Stenotheca, which we refer to the Patellacea, appears 
at all likely to have been concerned in their evolution. In this genus the shell is 
more or less strongly curved, the surface frequently cancellated, and the dorsum 
sometimes subangular. From such a type it is not very far to Cyrtolites. But 
Cyrtolites, despite the fact that it is closely connected, by one species or another, to 
more typical members of the suborder, is very different from Ovwenella, which 
includes the only Cambrian bellerophontid known. Nor has the genus yet been 
found in rocks older than the Trenton. Although, in placing the Bellerophontacea 
* Among recent Gastropoda only the Polyplacophora and Ophisthobranchiata are fundamentally symmetrical, but paleon- 
tology seems to show conclusively that the early or Paleozoic representatives of the Docoglossa, Pteropoda and Scaphopoda 
were perhaps equally so. 
