EEPOKT ON THE MAKSENIAD^E. 



arrnatum. Radida linearis, valida, postice in vaginam libere prorninentem, longam, 

 ssepins quasi spiraliter involutam continuata. Armatura radulae mire diversa. Dens 

 medianus semper solitarius, margine basali posteriori bifido vel truncato ; apice recurvato 

 cum acie unicuspide et utrinque denticulata. Dentes pleurales nurnero varii; semper 

 lateralis internus validus, interdum prseterea uncini duo externi. 



Podarium oblongum, antice truncatum, subemarginatum, margine antico sulco exara- 

 tum ; postice nonnibil acuminatum, apice subrotundatum (operculo lobuloque operculigero 

 nullis). 



Animalia pro parte dioica, pro parte monoica. Penis validissimus, pone tentaculum 

 dextrum exsertus, sat compressus ; interdum falcatus, apice incurvatus ; interdum involu- 

 tus ; sub inertia intra marginem cavitatis brancbialis recondendus. Apertura genitalis 

 feminina, vulva, verruca perforata intra angulum dextrum fissuras brancbialis sita. 



Animalia marina, ut plurimum solitaria, segnia, ova in cavitates Ascidiis compositis 

 vel Spongiis excavatas deponentia. Embryones testa externa provisoria carinis compluri- 

 bus iustructa. 



Tbe Marseniadse have a somewhat round or oval contour, and are generally almost 

 hemispherical. The dorsal surface {p>allium) is either smooth, or covered with a variable 

 number of large, bluntly conical or more spherical tubercles. The shell is either altogether 

 enveloped in the mantle, or is partially exposed at a round or oval opening on the middle 

 of the back (Marsenina). It covers the whole of the respiratory apparatus [Onchidiopsis), 

 and usually the whole of the upper visceral mass ; at the edge of the wide opening it is 

 intimately united to a rather thick flat band, the shell-fillet. The shell may have a shield 

 shape, without a trace of spiral-turning, and is then of a horny nature [Onchidiopsis) ; or it 

 may be almost horny and with a hint of a spiral [Marseniella) ; usually, however, it is wound 

 in a small spiral with few turns, of which the last is very wide, and in this case the shell is 

 always calcareous, but very delicate and brittle. On tbe inner side of the last turn of the 

 shell there are usually two (frequently indistinct) muscle-facettes, one somewhat anteriorly 

 to the left, the other posteriorly to the right. The border of the mantle is very well 

 developed, projecting all round, and usually completely covering head and foot, with the 

 exception of the tip of the tail. It is usually flattened and surrounds the upper portion 

 of the body as a border of varying breadth ; occasionally it is thicker, rounded at the 

 margin, and somewhat flattened on the lower side [Marseniopsis, Onchidiopsis). There 

 is no proper, markedly protruding siphonal structure ; but this is replaced by a semi-canal 

 on the under side of the mantle, which twists the border upwards in a protruding fold, or 

 ends between two finger-like prolongations (Chelyonotus). This semi-canal ends at the 

 left corner of the anterior end of the gill-cavity. Besides this inhalent canal there is in 

 some genera {Marsenina, Onchidiopsis), on the right side, a perfectly similar exhalent 

 canal, ending at the right corner of the branchial cavity. The branchial cavity is wide 

 and spacious, but depressed. On its roof are fixed the two so-called branchiae, the anterior 



