4o6 



DIOTOCARDIA— RHIPIDOGLOSSA 



CHAP. 



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. Fissurellldae. — 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.) Fissurellinae. 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^ Cli/pidella. (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- 

 surelUdaea^ PupiUaea., Lucaplna^Mega- 

 tebeimus, Macroschisma, Lucaphiella. 

 (iii.) Emarginulinae. Shell usually 

 wholly external, apex usually not re- 

 moved by perforation, sometimes with 

 internal septum, anal tube in a narrow 

 Genera: Gh/pJiis, externals of Fissiwella, but hole- 



FiG. 268. — Scutus ai(st7'alis Lam. 

 Australia: m, m, mantle; sh 

 shell. X §. 



slit or sinus 



callus truncated behind; PiuK'tiirella (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; Subemargmula^ 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 rather flattened, spire short, last 

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



