THE ANATOMY OF THE HI YER-MU SSEL. 
255 
the striated inner surface of its inner leaflet. The situation of 
the “ organ of Bojanus” is indicated by the letter r ; its dark 
glandular portion appearing through the thin texture of the 
mantle. 
On the nacreous inner surface of the left valve (V) of the 
shell, rendered visible by its disengagement from the left lobe of 
the mantle (m'), are seen the following markings : — a a' and 
p a' are the impressions of the former attachments of the 
anterior and posterior adductor muscles respectively. From one to 
the other runs a curved line ( p l) more or less parallel with the 
ventral edge of the shell, which is termed the 11 pallial impres- 
sion,” in that it marks the attachment of the inner boundary — 
not the free edge — of the 11 collar ” of the mantle. In Lamelli- 
branchs having retractile siphons, e.g., My a and Pholas, a kind 
of bay or re-entering angle is formed between the hinder curve 
of the pallial impression and the scar left by the posterior 
adductor ; this, which lodges the retractor of the siphon of its 
side, is called the “ pallial sinus.” A scar (p p), situated at the 
ventral side of that of the anterior adductor, and a few smaller 
markings (r p a) which are visible just in front of the umbo (u), 
indicate the former attachment of the protractor and anterior 
retractor muscles of the foot respectively. The faint lines which 
run parallel with the pallial impression and concentrically with 
the umbo ( u ) are the lines of growth, which mark the former 
limits of the shell valve. As many of the characters of fossil 
shells are determined by the aid of certain of the impressions 
just described, the affinities of those shells are not easily made 
out which have, through lying in porous chalky beds, had all 
the nacreous lining dissolved out, save when a cast of the 
interior has been formed before this came about. The letter h 
points to the inner of the two ridges bounding the groove in 
which the “ cartilage \ or spring (c) of the hinge is lodged. 
Figs. 2 and 3 are diagrammatic drawings from two injected preparations in 
the Oxford University Museum. A vertical longitudinal section 
has been made through the foot mass (f), in order to display the 
course of the digestive tube, which is seen both on its right 
(fig. 2) and left (fig. 3) aspects, a a and p a, anterior and 
posterior adductors in section r p p, retractor pedis posterior, 
divided close to its insertion ; s t points to the position of the 
“ crystalline style ” ; v h, ventricle of the heart seen in section ; 
r, “ organ of Bojanus ;” r g and l g, right and left gills. Where 
the intestinal tract swerves — in the first instance (fig. 2) to the 
left, in the second (fig. 3) to the right side — a bristle may be 
seen passed into the diverging portion, the course of which is 
further indicated in the figures by dotted lines. 
Figs. 4 and 5 illustrate the structure and relations of the u organ of 
Bojanus.” 
Fig. 4 is a semi-diagrammatic section (“ coupe un peu theo- 
rique ”), after Lacaze-Duthiers, of the organ in TJnio pictoruin 
(Ann ales des Sciences Naturelles , 4th ser. ; Zoologie , tome iv. 
