118 TRANSACTIONS OF THE [MAR. 17, 
‘this species as recorded in its shelly covering are very instruc- 
tive. Beginning with a shell which is comparatively tumid in 
form and nearly semicircular in outline, it finally becomes or- 
bicular in outline, and with valves flattened to the form of 
saucers. 
By the varied sculpture, the outlines of the valves and the 
surface markings, several phases in the life of this brachiopod 
may be distinguished. 
(1.) The first is that marked by the embryonic shell. This 
shell, now preserved in the umbo of the adult, shows in the 
markings on its surface faint indications of additions to its size, 
but these are hardly discernible. A remarkable feature about the 
embryonic shell is the form, which is entirely different from that 
of the adult, for it (in the dorsal valve especially ) is nearly semi- 
circular in outline, and is quite tumid when compared with the 
adult shell; it looks more like an Orthis or a Linnarssonia (see 
figs. la to c) than an Obolus or a Lingulella, the two genera 
which the adult most nearly resembles. The embryo ventral 
valve also differs quite as much from the adult as does the dor- 
sal, for in its high umbo and straight hinge line it recalls spe- 
cies of the genera Acrotreta and Kutorgina (see figs 2a to c). 
In the embryonic shell of the dorsal valve, which is narrowly . 
semi-circular, the straight outline of the hinge was scarcely 
broken by the slight, rounded projection of the umbo. As 
viewed from above, this valve presents a hollow more or less ob- 
vious in the front of the visceral cavity. This hollow is some- 
times a deeper depression, and corresponds to the outside of a 
tubercle or ridge within the shell, which continued throughout 
the life of the occupant to be a marked prominence of the interior 
of the valve on the mesian ridge of the shell near the hinge. The 
longest diameter of this hollow is about equal to one-half of the 
length of the embryonic shell, and in some examples it contains 
four little pits, which appear to mark the points of attachment 
of muscles (fig. ld). The two lateral pits appear to answer to 
the anterior adductors, and the anterior pit to the anterior re- 
tractor. The anterior adjustors in this stage of growth seem to 
have been at the front margin of the shell, and outside of the 
large depression above referred to, fora series of pits can be 
traced on the adult shell from the margin of the embryonic shell 
well out toward the outer edge of the ‘valve (fig. 1f). The pos- 
terior adjustors are probably. indicated by a depression on each 
side of the umbo. 
The space occupied at this time by the visceral cavity was 
large in prqportion to the size of the shell, and extended quite 
out to the margin; and no indication of the existence of a mar- 
