436 CRETACEOUS GASTROPODA 
is only indicated by a posterior indistinct furrow, and another one dividing it 
in two halves; next to it below is the foot, which is in front and often also 
on the sides somewhat expanded. The radula is short and composed of five longi- 
tudinal series of small transparent teeth, the mouth being besides usually provided 
with jaws and with a various number of tentacular appendages, occasionally of 
different length and thickness. The digestive organs lie immediately posterior to 
the head and extend backwards. There is no special central organ or heart 
present regulating the circulation of the blood, and it is impossible to distinguish 
between the arteries and the veins; there is, however, near the anus a strong 
muscle present, which at least supports the motions and vibrations of the blood. 
Special gills are wanting, and the respiration can take place through any portion 
of the body which comes in contact with the water; it is, however, probable 
that some places are more adapted to that purpose than others. Clark con- 
siders the symmetrical liver as gills, and says that the water enters to them through 
the posterior opening, thus making the resemblance to the Pelecypoda very close. 
Eyes are wanting, but the nervous system is otherwise rather complete and resembles 
some other Gastropoda. The sexes are distinct; the genital organs are placed 
symmetrically at the back of the posterior end; they are, however, in both very 
similar, consisting of three or more rows of blind sacks; the males have no external 
copulative organs; the fructification of the ova takes place through the water. 
The embryo has a very thin, usually somewhat inflated shell which generally 
is soon thrown off, being rarely retained and connected with the old one. The 
larva moves about with cilia, the body becomes gradually prolonged, the head 
surrounded with a number of appendages is indicated and a new tubular shell 
formed; thus the larva gradually more and more assumes the form of the old 
animal. 
From these remarks the general relations of the Dzyrazip to other Molluscs 
can easily be traced out. Lacaze-Duthiers proposed for them the name SoLENo- 
CONCH®, considering the same as a third order of the ACEPHALOMALACIA. Sars 
has drawn attention to the similarities of the tentacles in the Dzyrazupz to 
those of the Cephalopoda. Bronn places his ProsopocrpHaLA intermediate as 
between and equivalent to the Pelecypoda and the Gastropoda. We have already 
remarked—and it is perfectly clear from the known anatomy of the animal— 
that the great inclination to the bilateral and symmetrical development of the organs 
places the PRosoPocEPHALA very close to the Zubicola of the Pelecypoda, and con- 
sequently Bronn’s intermediate classification of the order appears to have very much 
in its favor. Still in comparing the structure of the shell of the PRrosopocEPHALA 
with that of the Gastropoda, the presence of a foot being adapted for locomotion 
and placed below the head, farther, the radula with its teeth as compared with those of 
Lectura, Lepeta and others, it appears to us that the PRosopocePHata partake of the 
principal distinctive characters of the Gastropoda and ought, therefore, to be regarded 
as the lowest type of that class, equivalent to the Preropopa or the POLYPLACOPHORA 
and other orders. We know from different other forms of Gastropoda, like Rhodope, 
and several other GyYMNOBRANCHIATA that the gills are occasionally perfectly absent ; 
