42 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
to indicate that the two were, to a certain extent, independent. For 
practical purposes, however, both may be referred to collectively as a 
single platform. 
The extreme development of the median septum above the plat- 
form seems to be for the sole purpose of supporting the jugum. Simi- 
lar septa occur in the genera Kayseria and Anoplotheca but in these 
cases, on the ventral surface of the saddle which passes over the sep- 
tum, a stem is developed that rests against the ventral or pedicle valve, 
so that all three taken together, septum, saddle, and stem, form a brace 
between the two valves. In Camarophorella it is clearly only a sup- 
port for the jugum. This is the only occurrence of the kind recorded 
for the subfamily Meristellinae, but the function of the median septum 
in Dicamara and Camarospira has not yet been determined. It is 
possible that the same relation may have existed there. 
The hrachidium} — The crura, as just stated, take their origin on 
the anterior margin of the hinge plate close to the dental sockets. They 
are well developed, moderately long, and curve slightly toward the 
beak. The primary lamellae of the brachidium come in contact 
with them at a sharp angle (pi. 7, fig. 14) but the evidence that the 
two were connected in the adult is slight although they almost certainly 
must have been. Three specimens have been observed showing the 
crura unbroken but with the brachidium entirely or partially broken 
away, and four jugal loops in which one or both primary lamellae are 
in perfect condition to the point of supposed attachment to the crura. 
Another shows the two structures in conjunction but not united. The 
crura are cylindrical throughout the half or third of their length above 
the base, but the free or anterior ends are flattened into a broad blade. 
In all of the specimens the ventral surface of the crus is very distinctly 
scarred or grooved as shown in figures 42 and 43 (pi. 8). In no case 
where the blade is well preserved is any fracture to be seen. Of the 
four specimens in which primary lamellae are preserved to the point of 
1 Terms descriptive of the brachidium are the same as those used by Hall and Clarke 
('95, p. 86). Their diagram of the jugum of Athyrls and an outline of the same structure 
in the form under consideration are given in figures 1 and 2 (pi. 6). On these the parts 
described are indicated by letters. In describing the jugum the writer has found it 
necessary to use certain terms as dorsal and ventral, anterior and posterior, to indicate 
the structures. Although this may not seem to be according to the customary usage of 
these terms, it is based on the orientation of the jugum in the shell, as will be better 
understood by referring to figure 14 (pi. 7). The dorsal end is the end that is attached 
to the primarj' lamellae; the ventral end is the opposite or free end, so called because 
they are directed respectively toward the dorsal and ventral valves. The anterior and 
posterior surfaces, likewise, are directed toward the anterior or posterior ends of the shell. 
