XIV DIOTOCARDIA — RHIPIDOGLOSSA 405 



half supra-intestinal, left half infra-intestinal ; heart usually in 

 front of the branchia (ctenidium), which is generally single ; 

 head with a single pair of tentacles ; animal dioecious, usually 

 marine, more or less contained within a shell, operculum 

 generally present. Cambrian to present time. 



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

 in the Bocoglossa and Helicinidae)^ branchiae bipectinate, front 

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

 (except in Neritidae) ; nervous system not cencentrated ; 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 : JPectino- 



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

 Acmaea (^= Tectura^, with. suh-genev?i Qollisella 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.) Patellinae. — Three lateral teeth on each side, two of them 

 anterior. Principal genera : Patella^ branchial circlet complete; 

 chief sections Patella proper, Scutellastra^ Ancistromesus (J.. 

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

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

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

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

 chial circlet interrupted in front. 



(5) Rhipidoglossa (p. 225). — Ventricle of the heart trav- 

 ersed by the rectum (except in Helicinidae), one of 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 



