BRACHIOPODA. 59 



1882. Meristella. Whitfield. Geology of Wisconsin, vol. iv, p. 321, pi. xxv, fig-. 5. 



1883. Meristella, Hall. Transactions of the Albany Institute, vol. x, p. 71. 



1889. Merisi'ma, Nettblkoth. Kentucky Fossil Shells, p. 102, pi. xxxiii, iigst. U), II. 



1889. Meristina, Beecher and Clarke. Memoire N. Y. State Mus., ]>p. 67-70, p]. vii, figs. 4-13. 



Di.iGNosis. Shells usually of small size ; valves subequally convex, ovate 

 or elongate in outline. Umbo of the pedicle-valve not high or greatly incurved, 

 usually exposing the circular apical foramen, beneath which the deltidi.al plates 

 are frequently retained. Cardinal slopes of both valves broad and not distinctly 

 defined ; anterior margin subtruncate and gently sinuate. In the typical forms 

 there is a faint sinus on both valves near the anterior margin, otherwise the 

 surface is smooth. On the interior the muscular impressions of the pedicle- 

 valve are similar to those of Meristella. In the brachial valve the hinge- 

 plate is concave, divided by a deep central concavity which is supported by a 

 median septum. On either side are the lobes bearing the bases of the crura. 

 The brachidium* consists of two spiral cones arranged as in Merista, but as 

 a rule the ribbon makes fewer (from six to twelve) volutions at maturity. 



FIG. 43. Fig. 44. 



The loop of Whiifieldella nitida. Hall, (c.) 



The loop is simple, the branches being more nearly erect than in Merista, 

 Meristella, etc., and beyond their junction continued into a short, acute, gener- 

 ally slightly curved process, which makes a large angle with the direction of the 

 lateral branches. The muscular impres.sions, which are very fiiint, are divided, 

 longitudinally, by the median septum, and, transversely, into anterior and pos- 

 terior scars. From the ante-lateral margins of the muscular area in both valves, 

 radiates a series of vascular sinuses, the principal trunks of which are very con- 

 spicuous ; this feature, however, is rarely retained. External surface of the 

 valves smooth or concentrically striate. Shell-substance fibrous, impunctate. 

 Type, Atrypa nitida, Hall. Niagara group. 



* The term bracMcUum may be applied to the calcified brachial supports of all Brachiopoda. 



