324 Mr. W. Clark on the Animal of Dentalium Tarentinum. 
corresponding ingress and egress of the water seen*. I have not 
detected auricles on each side of the heart, nor near it, as might be 
expected from the symmetry of the branchiz ; there are certainly 
minute points on each side of that organ, but I demur to call 
them auricles, and rather think they denote the valvular appen- 
dages of the heart to prevent regurgitation into the branchial 
veins. The blood of the posterior part of the body is brought 
to the branchial artery which runs at the inner base of the 
branchiz, by two longitudinal veins, which pass between the 
branchiz on their convex surface, receiving tributaries ; I could 
not trace those of the anterior part; the arterial blood is then 
distributed imto the ramifications of the branchiz, and after 
aération is passed by each principal vein, which coasts the edges 
of those organs at their dichotomous points, to the heart, which 
throws out a posterior and anterior short trunk, both of which 
bifurcate into two smaller arteries, which supply veins infusing a 
renewed vitality into all parts of the body, from whence the 
blood is again returned to the arterial centre. Under the micro- 
scope the blood of the tributary and superficial vems appears to 
be in some individuals of a pale pink colour, and in others of a 
purplish pale red cast. I have preparations to illustrate this order 
of the organs. 
The branchiz are two symmetrical, sublateral, and somewhat 
post-centrally situated, dark greenish brown, elongated, suboval 
organs, having their bases fixed on and hanging from the con- 
cave surface of the animal with their points vertically parallel to 
the bases ; the two branchiz are united at their inner surfaces 
by a bridle of branchial strands arranged symmetrically. 
The heart in the testaceous Gasteropoda, spiral and otherwise, 
is always placed at the posterior end of the branchial cavity, or, 
in other words, is fixed at that extremity of the branchiee furthest 
from the entry of the aérating fluid: this statement of position 
is of importance in coming to a conclusion as to the mode of 
entry of the water. But if the position of the organs of Denta- 
lium is examined under the view of the water approaching the 
branchize under the mantle, as in the ordinary Gasteropoda, they 
will be found to be the reverse of what I have stated to be the 
usual natural position ; the heart will be found at the anterior 
end of the branchial cavity instead of at the posterior, and near- 
est to the entrance of the water instead of furthest from it: here 
is a subversion of the order of nature im respect to the position 
of these essential organs : how are they to be placed in harmony 
with her laws ? The solution of this question 1s simple : we have 
* Lamarck in the last ed. ‘ Anim. sans Vert.,’ Milne Edwards’s, 3rd vol. 
p. 13, says, ‘Car, aprés les animaux vertébrés, la nature n’offre, dans aucun 
