NoetUng on the Pclecypods. — Riiedcmann. 39 
tweeii the Loxogoniacea (sinnpalliate Dimyaria) and the 
Svmptogoniacea ( Monomyaria ) , which in this diagram have 
become adjoining groups; while each of them is in the other 
direction connected with the next adjoining group; the Symp- 
togoniacea being generally considered as derived from the 
Opisthogoniacea, and the Loxogoniacea being closely related 
to the Orthogoniacea. Altogether there seems to exist from 
the Loxogoniacea in a direction against the movement of the 
hands of the watch a continuous series of groups, and from the 
Symptogoniacea with the movement of the hands of the watch. 
This leads to the conclusion that the two, Loxogoniacea and 
Symptogoniacea, represent the termini of two series, which 
are thought to begin with the Protoconchae. In the diagram 
this supposed phylogenetic connection is indicated by the circle 
which beginning at the Protoconchae is, in the upper semi- 
circle, marked as the phylum of the Dimyaria, in the lower as 
that of the INIonomyaria. 
The problematic group of the Protoconchae which would 
represent the primitive pelecypods, can be said to possess the 
following characters : Equivalve, but inequilateral. Two mus- 
cular impressions. Pallial line entire, without sinus, situated 
upon the umbonal side of the shell, i. e. with its convexity di- 
rected toward the umbo. Hinge consisting of primary lamel- 
lae. Ligament partly external, partly internal. Schalenschiefe 
between 180' and 270°, and growth in oro-dorsal direction. On 
the whole the shell would be like that of a common dimyarian, 
but the animal lay in it turned 180°, so that the ventral organs 
were situated in the umbonal direction. It is stated that this 
definition of the Protoconchae tallies in the more important 
features with that furnished by Dall in Zittel-Eastman's text- 
book of the Protopelecypoda, and derived from the characters 
of the prodissoconch. The diffi,culty of recognizing this im- 
portant group among the paleozoic pelecypods is conceded, and 
it is added that only the pallial line and the direction of growth 
will definitely determine the existence of this group. 
The author thinks that these groups could be also used for 
systematic purposes, specially as several coincide in the whole, 
with classes of the original system of pelecypods. It would 
seem, however, to the writer that the groups can not be of 
equal value, as some represent only a single momentary stage 
