24 THE LIFE OF THE MOLLUSCA 
say % 
Order I.: AspPrpoBRANCHIA, having the gill 
filaments flattened out into leaf-like expansions. 
Sub-Order 1: Dococtossa (Plate IX., Figs. 7-9; 
Radula, Plate III., Fig. 12), comprising the Limpet 
family. 
Sub-Order 2: RHIPIDOGLossA (Plate IX., Figs. 
10-24; Radula, Plate III., Fig. 13), comprising the 
Ormers (Haliotide), Keyhole Limpets (Fissurel- 
lidze), etc., in which the older members still retain the 
right (originally left) gill, while the more specialized 
have only one, the left (originally right) gill. 
Order II.: PECTINIBRANCHIA, having elongated 
gill filaments. 
Sub-Order 1: T#NrIoGLossa (Plate X.), which 
includes the greater part of the sub-class, and in 
which the radula formula is generally 2:1:1:1:2 
(Plate III., Fig. 15). 
Sub-Order 2: STENOGLOSSA. 
Tribe 1: RuAcHIGLOssA (Plate XI.), including 
the Whelks, Dog-Whelks, and their kind, and 
having a radula formula of 1:1: 1 (Plate III., 
Fig. 17), although in higher members of the tribe 
this is reduced to 1 (Plate III., Fig. 18). 
Tribe 2: ToxocLossa (Plate XII., Figs. 1-5), of 
which the Cones and Auger Shells (Tevebra) are the 
best-known examples. The radula formula is 1: 0:1 
(Plate III., Fig. 19), or exceptionally 1: 1: 1. 
Sub-Class 2: The EuTHyNneurA fall into two 
orders and four sub-orders : 
