396 CRETACEOUS GASTROPODA 
Tn the following pages I shall accept the sub-division of the OPISTHOBRANCHIA, 
according to Bronn,* into PLEUROBRANCHIATA and NoroBraNncHiata, the former 
including the species in which the gills are more or less covered by the mantle, 
and the latter those in which they are uncovered, being exposed freely or repre- 
sented by a simple ciliated surface of the back. These two divisions are equivalent 
to those of Philippi’st Tecti- and Nup1-Brancuta, or to H. and A. Adams’ Trcrt- 
and NUDI-BRANCHIATA,{ but the names introduced by Bronn appear more correctly 
applicable. 
The Noroprancuiata are all naked Mollusca, and, therefore, very unfavorable 
for preservation in a hard rock. There have not yet been any fossil species 
reported, but the possibility of such being found in the fossil state is sufficiently 
exemplified by the discovery of larvee of insects and even medusve in the litho- 
graphic slates of Solenhofen, animals which often have a much softer body than 
many of the Noroprancutata. Bronn divides this sub-order into five tribes, 
Dermato-, Puaco-, CeraA-, Ciapo-, and PyGo-BRANCHIATA, and these again into a 
number of families. Not having any fossils from the cretaceous rocks of South 
India to report as belonging to any of the divisions of the NoTroBRANCHIATA we 
shall not enter farther into any discussion on their organisation. 
Sub-order, Pleurobranchiata, 
Char. Opisthobranchia with gills consisting of a tuft, a uni- or bi- serial plume, 
or of numerous lamelle, generally unsymmetrical, but always more or less covered 
by the mantle, the latter usually secreting a@ lamella-like, cup-shaped, or spiral 
shell. 
There are two divisions made in this sub-order, called DiptEvRo- and Mono- 
PLEURO-BRANCHIATA. The former have the gills consisting of numerous lamelle, 
placed almost symmetrically on either side under the edge of the mantle, some- 
what similar tosome of the PoLypLAcopHora; they are separated only into two 
families, Payzzipipz# and PrevropHyzLpip®, the animal of both having a some- 
what leathery skin, but none or a very rudimentary shell. We shall, therefore, only 
notice the second division. 
Tribe, Monopleurobranchiata, 
The species included in this tribe have the branchial plume,—with the excep- 
tion of Posterobranchea, D’Orb.,—placed on the right side; there are all gradations 
in the development of the shell to be met with, it is rarely wanting, sometimes 
imbedded in the mantle, but more usually external, symmetrical in the Uwerezzip2, 
flat and small in the Apzyszrpz, spiral in many other families, in the AcrmonipE 
so large as to cover perfectly the animal when retracted. 
* Klassen und Ordnungen, III, p. 795. 
+ Handbuch der Conch., 1853, pp. 222 and 265. 
+ Troschel generally accepts a three-fold division in the order of the OPISTHOBRANCHIA, namely, NorTo-, 
MoNnoPLEvURO-, and HYPO-BRANCHIATA. 
