330 PATELLID A. 
a large, simple rachidian tooth with, on each side, two large, 
simple, transverse laterals, followed by two minute ones, and a 
large outer lateral with a strong tridentate cusp, outside of which 
is a single scale-like flat uncinus, bearing an elongated, thickened 
ridge, but no cusp. 
For this genus Mr. Dall has proposed a new family ; he states 
that it might be incorporated with Cocculinidee, ‘‘ were it not for 
the differences in the branchiz and in its dentition.” 
Famity PATELLID A. 
Shell wholly external, dish-shaped, with apex anteriorly 
directed; animal with two short tentacles, a non-extensible 
muzzle; branchiz external or none; renal and anal aperture 
situated above the neck, between body and mantle-edge; no 
copulatory or external genital organs; mouth provided with 
horny jaw, and long radula with numerous peculiar black, 
opaque teeth, and pellucid or colored plates or bosses; meta- 
morphosis of the embryo taking place in the egg, which i is fer- 
tilized in the ovary. 
The Limpets have been very thoroughly studied by Mr. Wm. 
H. Dall,* who has proposed an elaborate classification of them, 
including ordinal and subordinal as well as family and generic 
characters. JI have mainly followed Mr. Dall’s system in the 
diagnoses and sequence of the groups, but without giving them 
the same systematic values :—for example, I have used above his 
characters of the Order Docoglossa for the family Patellide, 
thus making the family more comprehensive than in his sense, 
and corresponding more nearly in conchological importance with 
the other families in this work. Similarly Mr. Dall’s suborders 
correspond nearly with my subfamilies and his family charac- 
ters are here treated as generic. The order Docoglossa is, as its 
name implies, founded upon peculiarities in the arrangement of 
the lingual dentition (xii, 51), but already two forms of limpets 
have been discovered which by their dentition cannot be placed 
in this order. Cuvier united the Patellide and Chitonide in his 
order Cyclobranchiata, characterized by the arrangement of the 
gills in a circle surrounding the body, but more recent investiga- 
tors have ascertained a considerable diversity of gill-arrangement 
among the limpets, so that this term will no longer apply to 
them as a whole, although many of them agree with the Chitons 
in this feature. 
SupraMiILy LHPE TINA. 
. 
Animal without branchie. Embryonic shell spiral. 

* See his papers in Am. Jour. Coneh., AL vi, and Proceedings of thé 
National Museum. 
