154 PALEOZOIC PALEONTOLOGY. 
The dimensions of a nearly-perfect specimen are: length, 9.5 mm. ; 
width, 12 mm.; thickness, 7 mm. 
Remarks.—As it occurs in the Trenton limestone of New Jersey, 
this species never grows to the large size which it frequently attains 
elsewhere. ‘The largest specimens observed do not exceed 20 mm. in 
breadth and but few specimens exceed 15 mm. The range of varia- 
tion among the New Jersey specimens is not great, and the above 
description has been drawn up to accord with these as nearly as 
possible. 
DINORTHIS PECTINELLA (Emm.). 
Plate IX., Figs. 29-30. 
1892. Orthis (Dinorthis) pectinella Hall and Clarke, Pal. N. Yigvel: 
WAL opin aS pliaaen 
gs. 27-33. 
Description—Shell resupinate, transversely subelliptical in outhne, 
wider than long, in about the proportion of four to three; cardinal 
line usually less than the greatest width of the shell, the cardinal 
extremities rounded; surface of each valve marked by from twenty- 
two to thirty prominent, rounded, simple coste, which are equal in 
width to the spaces between, and are crossed by fine, closely-crowded, 
elevated, concentric lines of growth. Pedicle valve slightly convex 
near the beak, flattened on the sides, with a broad, shallow, ill-defined 
depression along the centre, usually most distinct in front, but fre- 
quently nearly obsolete. Cardinal area moderately large and well 
defined, flat, lying nearly at right angles to the plane of the shell. 
Brachial valve regularly convex, most prominent in the centre, flat- 
tened and slightly deflected near the cardinal extremities. Cardinal 
area much narrower than that of the opposite valve, lying nearly in 
the plane of the shell. 
The dimensions of a nearly-perfect pedicle valve are: length, 21 
mm. ; width, 27.5 mm. 
Remarks.—This species is almost always imperfectly preserved in 
the Trenton limestone of New Jersey, and it is often next to im- 
possible to distinguish specimens of the brachial valve from Plectorthis 
plicatella. There is no difficulty, however, in recognizing the nearly-« 
flat pedicle valve. 'The species is characteristically a low Trenton 
form. 
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