328 CRETACEOUS GASTROPODA 
they pass one into the other. As soon as we know more of the form and the 
position of the gills in the numerous and different species of the CycLOBRANCHIATA, 
it may very likely be shown that the distinction of the two last sub-orders is not 
necessary, but there are many difficulties as yet to be overcome. Gray proposed the 
name HETERODONTA and Mérch that of Orraoponta; Troschel introduced the 
name Docoeiossa including in it, beside this and the former sub-order, also the 
CHITONIDA or PoLYPLACOPHORA and the DEntTaLitp® or ScapHoropa, (vide Archiv 
fiir Naturgesch., 1866, XXXII, p. 257). 
A more detailed description of the form of the animals, their dentition, &c., 
will be found in the treatises of H. and A. Adams, Gray, Troschel and others. 
XLVI. Family —PATELLID LA. 
Having excluded the PoLtypLacopHora and the ScapHoropa there only remains 
one family to be mentioned, and several conchologists even object to recognise 
more than one genus, Patella. It is well known that in all the species of 
Mollusca, which generally live attached to foreign objects, small variations in 
the form of the shells occur, and that therefore the exact form of the shell 
has not the same generic value as is the case in other free-living species. Still 
when a portion of the shell is regularly produced to so great an extent as in 
the Pat. cochlear, Gmel., a generic or sub-generic distinction appears desirable. 
Gray quotes four genera, two of which, Nacella and Helcion, have been placed 
in the Zzcruripz. 
1. Patella, Linn., 1752. Shell depressedly conical with suborbicular or oval 
aperture, sub-anterior apex, and externally usually covered with radiating ribs, which 
project more or less on the margin (vide antea p. 321). There are only a few finely 
striated species known, and even regarding these it is not quite certain whether 
they do or do not belong to the Tzcruripm. In fossil shells the presence of 
stronger projecting ribs is almost the only, at least approximately correct, distinction 
between Patella and Tectura. 
la. Olana, H. and A. Adams, 1855, proposed for Pat. cochlear, Gmel., 
having the anterior portion of the shell much produced. The animal is not different 
from those of other true Patelle; the shell resembles that of some of the fossil 
Metoptome (vide antea, fam. Gapryizpm). Nacella, Schumacher, (Patina, Leach 
apud Gray,) and Helcion, Montf., which have the gills composed of filiform 
strands not of lamellee and interrupted over the head, have been, as already 
noticed, placed in the Tzcervripz. 
The habitat of the Parzzz1pz is well known ; they are always found on rocky 
coasts. Fossil species occur, though not very numerously, through all the sedi- 
mentary deposits from the oldest to the upper tertiaries. Some palzeontologists, 
however, prefer to call the paleozoic and older secondary species by different 
names, and place them in different families. Pictet and Campiche, in their list of 
cretaceous species (Mat. p. 1. Pal. Suisse, 8me. Ser., Foss. Ste. Croix, p. 721), 
