XIV DIOTOCARDIA RIIIPIDOGLOSSA 405 



half supra-intestinal, left half infra-intestinal ; branchia (cteni- 

 dium) generally single, usually in front of the heart ; head with 

 a single pair of tentacles ; animal dioecious, usually marine, 

 more or less contained within a shell, opercukmi generally 

 present. Cambrian to present time. 



Sub-order 1. Diotocardia. — Heart with two auricles (except 

 in the Docoglossa and Helicinidae), branchiae bipectinate, front 

 end free ; two kidneys, the genital gland opening into the right 

 (except in Neritidae) ; nervous system not concentrated ; no 

 proboscis or siphon, penis usually absent. 



(a) Docoglossa (p. 227). — Heart with a single auricle, 

 ventricle not traversed by the rectum, visceral sac not spiral, 

 shell widely conical, non-spiral, no operculum ; radula very long, 

 with few hooked teeth in each row. 



Fam. 1. Acmaeidae. — Left ctenidium alone occurring, free on 



a long stalk. Cretaceous . Principal genera : Pectinodonta, 



front part of head much produced, radula (1. 0. 1.) ; Acmaea 

 ( =: Tectura), with sub - genera ColUsella and Collisellina, no 

 accessory branchial ring, shell closely resembling that of Patella, 

 but generally with a distinct internal border ; Scurria, accessory 

 branchial ring on the mantle. 



Fam. 2. Lepetidae. — No ctenidia or accessory branchiae, 



animal generally blind. Pliocene . Principal genera : 



Lepeta ; Propilidium, apex with internal septum ; Lepetella. 



Fam. 3. Patellidae. — No ctenidia, the osphradial patch at the 

 base of each alone surviving, a circlet of secondary branchiae 



between the mantle and sides of the foot. Ordovician . 



(i.) PatelUnae. — Three lateral teeth on each side, two of them 

 anterior. Principal genera : Patella, branchial circlet complete ; 

 chief sections Patella proper, Scutellastra, ATwistromesus (A. 

 mexicana Brod., measures 8-14 in. long); Helcion, branchial 

 circlet interrupted in front; TryUidium (Ordovician). — (ii.) 

 Nacellinae. — Two developed laterals on each side, one anterior. 

 Genera : Nacella, branchial circlet complete ; Helcioniscus, bran- 

 chial circlet interrupted in front. 



(&) Ehipidoglossa (p. 225). — Ventricle of the heart traversed 

 by the rectum (except in Helicinidae), one or two ctenidia ; jaw 

 in two pieces, radula long, marginals multiplied, rows curved. 



Of all the Gasteropoda, this section of the Diotocardia approach 

 nearest to the Pelecypoda, particularly in the least specialised 



