54 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The nuclei are large and spheroidal, measuring, in some cases, as much as 
0.008 mm. in diameter. The nuclei in the more superficial parts of the organ 
are much larger than those situated more deeply. 
In the centre of the thicker part of the cone are a number of scale-like 
bodies, placed at right angles to its axis (c. s.). There are from ten to twelve 
of these scales, measuring on an average 0.08 mm. in diameter, and 0.012 mm. 
in thiekness. In structure they resemble, on the whole, those forming the 
posterior cup. They stain in the same way and to the same depth, but they 
are, on the whole, somewhat thinner in proportion to their diameter, and the 
fibres composing them are more obvious. 
The form of this central mass constitutes the most important difference 
between the present species and P. margaritifera, for in the latter it is much 
shorter, the conieal part only extending halfway along the sides of the inner 
cup, and being hollowed to receive the deeper portion of a strueture I have 
called in that species the “internal cone.” 
5. The Anterior Cap (a. c.) covers the whole surface of the organ above the 
central mass. It has the form of a meniscus, the deeper surface, which rests 
upon the central mass, the inner cup, and the edge of the posterior cup, being 
deeply concave, whilst the upper surface is very convex. The greater part of 
it is composed of delicate wavy fibres interlacing with each other, and disposed 
in general parallel with the surface of the organ. Over the central mass the 
fibres are closer and more nearly parallel than over the lateral portions of the 
organ, where the texture is much more open, leaving rounded interspaces be- 
tween them, A few nuclei were seen in these lateral parts, but none in the 
centre. 
In the superficial layers of this anterior cap are a number of scales disposed 
parallel to the surface. They very closely resemble the scales of the outer 
layers of the posterior cup, but are somewhat larger in diameter and not quite 
so thick. They stain very deeply, and the more superficial ones are almost 
homogeneous in composition. There are a few of these scales in the deeper 
layers, but these are much smaller and show a tendency to break up into fibres 
something like the scales in the middle of the central mass. 
II. THE SIPHONAL ORGANS (Fig. 1, S. S.) are paired and lie just within the 
hinder margin of the funnel, and are seen peeping out from beneath it when 
the mantle-cavity is opened. Each organ is bluntly pyriform in shape, or 
more accurately, perhaps, has the form of an acorn in its cup (Fig. 5). The 
smaller end is directed backwards, and at the inner side of the broad end is 
a kind of stalk by which it is attached to the ventral wall of the body. The 
free distal end is slightly pigmented, while the broad end is paler and the tip 
is a paler patch of a semi-transparent yellowish horn color. The length of 
the organ is about 0.9 mm. and the greatest diameter 0.8 mm. 
In minute structure the siphonal organs closely resemble the ocular organs 
in their main features (Fig. 6). It will be sufficient therefore to enumerate 
their constituent parts and to mention the points in which they present differ- 
ences. On the whole, they are more like the organs of P. margaritifera. 
