548 GASTROPODA. [Opisthobranchia. 



Tribe 3. PLEUROBRANCHOMORPHA. 



Tectibranchs with 2 pairs of tentacles. The foot is devoid of 

 parapodia. There is no pallial cavity, but there is always a single 

 ctenidium situated on the right side, and occupying the space between 

 the mantle and the foot. The genital duct with 2 orifices, without an 

 open seminal groove ; the male and female apertures are contiguous. 



Fam. UMBRACULID-ffi, Pilsbry. 

 Umbrellidce of authors. 



Animal having the foot oval or oblong, adapted for creeping, 

 without pleuropodial process. Head bearing 2 laterally slit tentacles, 

 the eyes sessile at their inner anterior bases. Mantle the size and shape 

 of the shell, with thin serrate edge. Gill a long plume lying between 

 mantle and foot on the anterior and right side, adnate and bearing 

 numerous bipinnate branches for the greater part of its length, posterior 

 end free and bipinnate. Anus tubular, projecting behind the gill. 

 Mouth with labial tentacular or plate-like processes. Radula very 

 broad, bearing a great number of similar, very narrow, crowded, 

 needle-like teeth, with recurved simple cusps, which are not differenti- 

 ated from the body of the tooth. 



Shell external, limpet-like, with the nucleus minute and sinistral, 

 vertex near the centre ; inside with a circular closed muscle-impression. 



Distribution. World-wide in tropical and subtropical seas, lami- 

 narian zone and deeper. 



Genus 1. UMBRACULUM, Schumacher, 1817. 



Umbraculutii, Schumacher, Essai Nouv. Syst. Vers. Test., 55, 177. Type : 

 Patella umbella, Marty n. Patella sp. of the older authors. Acardo, 

 Lamarck (in part), 1801 ; not of Commervon, Bruguiere, Cuvier, Miihl- 

 feldt, or Swainson. Gastroplax, Blainville, 1819. Umbrella, Lamarck, 

 1819. Umbflla, d'Orbigny, 1841. Operculatum, H. and A. Adams, 1854. 



Animal having the foot very fleshy, large, oval, with a deep anterior 

 sinus in which the mouth-parts are situated. Gill a long adnate plume, 

 extending across the front and along the right side, free and bipinnate 

 behind. Verge external, lying in the anterior sinus of the foot, in the 

 median line in front of and below the head. Radula extremely wide, 

 composed of an enormous number of perfectly similar, very narrow, 

 needle-like teeth, strongly recurved toward their apices, the cusps 

 narrowly lanceolate and smooth. 



Shell patelliform, depressed, sinistral ; the vertex to the left of 

 and somewhat behind the centre, usually coloured, more or less conic- 

 ally elevated, apex curved backward, when perfect forming a minute 

 spiral of scarcely over one whorl. 



Distribution. The genus occurs in tropical and subtropical seas of 

 both hemispheres. 



Fossil in the Eocene of Europe and America. 



