4o6 



DIOTOCARDIA — RHIPIDOGLOSSA 



specialised forms. The auricle, the branchiae, and the kidneys 

 are in many cases paired, and more or less symmetrical. The 

 ventricle is generally traversed by the rectum, there is a long 

 labial commissure between the cerebral ganglia, special copula- 

 tive organs are usually absent, while the shell is often nacreous, 

 like those of Pelecypoda of a primitive type. 



Section I. Zygobranchiata. — Two ctenidia, shell with 

 apical or marginal slit or holes, corresponding to an anal tube 

 in the mantle (p. 265). 



Fam. 1. FissurelUdae. — Two symmetrical ctenidia and kid- 

 neys, visceral mass conical, shell conical, elevated or depressed, 

 with a single anterior or apical slit or impression ; no operculum. 



Jurassic . (i.) Flssurellinae. Shell wholly external, apex 



entirely removed by perforation, apical 

 callus not truncated posteriorly ; cen- 

 tral tooth narrow. Genera: Fissurella 

 (Figs. 171, p. 261; 178, p. 265), Fis- 

 suridea^ Clypidella. (ii.) Fissurelli- 

 dinae. Shell partly internal, otherwise 

 as in (i.) ; central tooth broad, mantle 

 more or less reflected over the shell, 

 apical hole very wide. Genera: Fis- 

 surellidaea^ Pupillaea^ Lucapina^Mega- 

 tebennus, Macroschisma^ Lucapinella. 

 (iii.) Fmarginulinae. Shell usually 

 YiG.2&^.-Scutusaustraiisi.^m., wholly external, apex usually not re- 



Australia: m, m, mantle; sh, , , r ,• ,• •,! 



gj^gjj ^ 2 moved, by perioration, sometimes witn 



internal septum, anal tube in a narrow 

 slit or sinus. Genera: G-li/phis, externals of Flssurella, but hole- 

 callus truncated behind ; Pimcturella (sub-genera Cranopsis and 

 Fissurisepta)^ slit just anterior to the apex, a small internal 

 septum ; Zeidora, large internal septum as in Crepidula : Emar- 

 ginula^ shell elevated, slit very narrow, on the anterior margin 

 (in subg. Rimula^ it is between the apex and the margin), radula 

 bilaterally asymmetrical; Suhemarginula^ margin indented by a 

 shallow groove ; Scutus (^= Parmophorus) shell oblong, depressed, 

 nicked in front, largely covered by the mantle. 



Fam. 2. Haliotidae. — Right ctenidium the smaller, epipodial 

 line broad, profusely lobed ; shell ratlier flattened, spire short, last 

 whorl very large, with a row of perforations on the left side, 



