198 jETHERIIDJE. 



For the anatomy of this interesting species it may be well to 

 quote from the late Mr. Martin F. Woodward's invaluable paper 

 on the subject : 



" The Mantle and Muscular System : The mantle folds are 

 almost completely separated from one another, being united at 

 two points only, viz., at the posterior attachment of the gills, 

 thus separating an iuhalent from an exhalent orifice, and dorsally 

 from the posterior end of the hinge-line for about one-quarter of 

 the distance between this point and the attachment of the gills. 

 The margins of the mantle below the attachment of the gills are 

 beset with small sensory papillae for a space which occupies about 

 one-quarter of the extent of the mantle-margin from the attach- 

 ment of the gills to the anterior hinge-line " 



" A small posterior pedal retractor is present in Mulleria, situ- 

 ated immediately above, or anterior to the posterior adductor, and 

 exhibiting the typical relations of that muscle to the organ of 

 Bojanus and to the nerve-cords." 



" The Mantle Cavity. The branchial cavity is large and entirely 

 open below. The supra-branchial (anal) chamber opens poste- 

 riorly by a wide slit; it consists of a large median chamber behind 

 the posterior adductor muscle, where it receives the opening of 

 the anus. Under the adductor this chamber becomes divided into 

 three by the union of the primary branchial filaments with the 

 body -wall on either side." 



" Still further forward, the middle of these three chambers 

 becomes divided by the intervention of the visceral mass. In the 

 relationship of this cavity to the surrounding parts, Mulleria 

 resembles typical Eulamellibranchiata ; there is, however, no 

 communication, other than through the gill-slits, between the 

 branchial and supra-branchial cavities, such as we meet with 

 in a form like Anodonta" 



" The external apertures of the organ of Bojanus, and of the 

 genital gland, open into the innermost subdivision of the supra- 

 branchial cavity." 



" The Foot and Visceral Mass. The muscular foot present in 

 most lamellibranchiata is entirely wanting in Mulleria, a condition 

 which is correlated with the fixed abode of life of this form .... 



" The visceral mass is small and slightly bilobed posteriorly ; it 

 exhibits a few superficial muscular fibres mainly connected with 

 the posterior pedal retractor." 



" The Labial Palps. Two pairs of very small leaf-shaped 

 labial palps are present ; they are equal in size, and exhibit 

 oblique ridges on the opposed surfaces. The inner pair unite 

 below the mouth to form the lower lip, while the outer pair, 

 which become slightly expanded anteriorly, unite above the 

 mouth/' 



" The Respiratory and Circulatory Systems : The gills of 

 Mulleria consist of two lamellae on either side of the body ; of 

 these the outer is slightly the smaller. Their relations to each 



