24 THE LIFE OF THE MOLLUSCA 



Order I. : Aspidobranchia, having the gill 

 filaments flattened out into leaf-like expansions. 



Sub-Order i : Docoglossa (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 (Haliotidse), Keyhole Limpets (Fissurel- 

 lidse), 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^enioglossa (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 : Rhachiglossa (Plate XL), 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 : Toxoglossa (Plate XII., Figs. 1-5), of 

 which the Cones and Auger Shells (Terebra) are the 

 best-known examples. The radula formula is 1 : : 1 

 (Plate III., Fig. 19), or exceptionally 1 : I : 1. 



Sub-Class 2 : The Euthyneura fall into two 

 orders and four sub-orders : 



