4o6 



DIOTOCARDIA RHiriDOGLOSSA 



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 com- 

 missure between the cerebral ganglia, special copulative 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. Fissurellidae. — Two symmetrical ctenidia and kidneys, 

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

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



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



entirely removed by perforation, apical 

 callus not truncated posteriorly ; cen- 

 tral tooth narrow. Genera : Mssurella 

 (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 tlie shell, 

 apical hole very wide. Genera : Fis- 

 surellidaca, Pupillaea, Lucapina, Mega- 

 tebennus, Macroschisma, Jjucapinella. 

 (iii.) Fttuirgimdinae. Shell usually 

 wholly external, apex usually not re- 

 niautie ; xii, movod byperforation,sometinies with in- 

 ternal septum, anal tube in a narrow slit 

 or sinus. Genera : Glyphis, externals of Fissurella, but hole-callus 

 truncated behind ; Fwncturella (sub-genera Cranopsis and Fissuri- 

 -srj:;^«), slit just anterior to the apex,a small internal septum; Zeidora, 

 large internal septum as in Crrpidula : Fmarginula, shell elevated, 

 slit very narrow, on the anterior margin (in subg. Fimula, it is 

 between the apex and the margin), radula bilaterally asymmetrical ; 

 S'id)einargi7iula, margin indented by a shallow groove; Scutus 

 ( = Farmophorus) shell oblong, depressed, nicked in front, largely 

 covered by the mantle. 



Fam. 2. Hcdiotidae. — Eight ctenidiuni the smaller, epipodial 

 line broad, profusely lobed ; sludl rather ilattened, spire sliort, last 

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



Fig. 268. — Scutus (mstralis Lam 

 Australia : vi, vi 

 shell. X %. 



