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. lento), 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 strias. Shell- 

 structure fibrous, impunctate. 



Type, Merista lavis, 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. 



