BRACHIOPODA. 
75 
within the anterior margins of the lobes thus formed. In some species the 
hinge-plate is more subquadrate in outline, the variation being produced by the 
development of post-lateral expansions. This plate is supported by a median 
septum, which extends for somewhat more than one-third the length of the 
valve. The crura are short and straight, and the primary lamellae of the 
brachidium originate from them at an acute angle, and come into closest appois- 
tion at the anterior extremity of the median septum. In the mature individ¬ 
ual, the spiral ribbon makes about fifteen volutions, the bases of the cones being 
subparallel to the longitudinal axis of the shell and their apices directed toward 
its lateral margins. In their general shape the cones conform to the character 
of the interior cavity, and in the less convex species {M. Walcotti, M. lenta), they 
are appressed on the side of the flatter or brachial valve. The structure of the 
loop is the same as described for the genus Merista, with this difference, how¬ 
ever; the circular arms of the loop curve first outward in the horizontal plane, 
then backward and abruptly downward to the inner edges of the primary 
lamellae; in their return the same curvature is reversed and they therefore meet 
the stem of the loop in the horizontal plane, their point of union being invari¬ 
ably above the point of coalescence of the lateral branches of the loop. 
The muscular area is elongate-ovate, and extends for the entire length of the 
median septum; the four adductor scars are sometimes distinctly seen, the 
posterior pair being broader and embracing the posterior extremities of the 
anterior scars. 
External surface of the valves smooth or with concentric striae. Shell- 
structure fibrous, impunctate. 
Type, Merista Icevi.s, Hall. Lower Helderberg group. 
Observations. The term Meristella was introduced in 1859,* in connection 
with a revised list of the fossils which had been described in Volumes I and 
II of the Palaeontology of New York. The species which had been designated 
as Atrypa naviformis, from the Clinton group, is there referred to as Merista ? 
naviformis, and in a footnote therefrom it is said; “ This species and some others . 
of the Clinton and Niagara groups differ somewhat from the true Merista; and 
* Twelfth Annual Report of the New York State Cabinet of Natural History, p. 78» 
