File:The Quarterly journal of the Geological Society of London (12733849603).jpg

T. DAVIDSON AND W. KING ON THE TRIHERELLID.E. 133
quires the pertaining muscle to have been divided at its extremity,
one division attached to the hinge, and the other to the umbonal
cavity. But another idea regarding the cardinal scar (v) has been
adopted by Dall, that it afforded attachment to a muscle homologous
with that belonging to the cardinal process of the Terebratulids.
Certainly from its position, and especially when seated on an eleva-
tion, as in Trimerella Lindstromi, this scar has much the appearance
of being identical with the process characteristic of the hinge in the
family just referred to

but we think a more correct appreciation of
its true relationship is involved in the suggestion that it is homolo-
gous with the small pair of postlateral or converging muscles, of
which, as in Discina, a member passes from near each side of the
pedicle- or flat valve to the space between the posterior adductor
muscles in the brachial or convex valve. This suggestion, it must
be understood, does not necessitate tbe abandonment of the idea that
the scar (w, w) belonged to the umbonal muscle

only, instead of
this muscle being divided, with a division implanted on each side of
the pair of converging muscles, as in Discina, it may have been
simple in some cases, and have passed between the latter muscles.
Such an arrangement would be the reverse of what prevails in Dis-
cina

but it does not seem to be opposed by any valid objections.
With respect to the slanting muscles of Lingula, we strongly sus-
pect that the transmedial pair was absent in the Trimerellids — the
close union of the valves at the hinge, or the possession of a dentary
apparatus, rendering impossible any lateral movement of the valves
in their posterior or umbonal region, such being the function of the
transmedials.
The three remaining pairs of slanting muscles, which have been
generically designated laterals, and severally named " anteriors,"
" middles," and " outsiders," appear to us to have characterized the
fossil family.
The " anterior " pair of Lingula may correspond with, in the Trime-
rellids, the anterior scars, n*, of the brachial valve, and the ends of the
crescent (s), of the pedicle one

the " middle " pair with the median
scars, m, of the pedicle-valve, and the ends of the crescent, s, of the
brachial one

the " outside " pair with the lateral scars, o, of the
pedicle-valve, and the ends of the crescent, s, of the brachial one.
In accordance with these suggestions, the letter s (that is, in the
brachial valve) denotes a compound scar; but, instead of being
triple, as in Lingula (which, it must be borne in mind, has, in addi-
tion, one of tbe terminations of the transmedial muscle implanted in
the corresponding part), it would be double.
Moreover, in Lingula anatina a pair of slanting muscles is im-
planted in the brachial valve, adjacent to, or upon the medio-
longitudinal ridge, a member on each side, where the scars left by
the muscles are generally well seen. We have already assumed these
scars to correspond with those lettered n in figs. 4, 6, that is, on the

We have not lettered that portion of the platform in PI. XII. fig. 2, where, ap-
parently the scars would be situated, as there are grounds, presently to be noticed,
for believing that they were in this instance transposed to the median plate