BRAOHIOPODA OF THE MARL BEDS. 13 



Terebratella plicata (Say) D'Orb. Prod., p. 259. Gabb, Synop. Gret. Foss., p. 193. 

 1861. Meek, Geol. New Jersey, 1868, pp. 375, 723. 



Shell of medium size, subcircular in outline and somewhat plano-con- 

 vex in profile, marked by from eight to twelve or more sharply an; alar 

 plications, which extend from beak to base. Ventral valve strongly convex, 

 with an erect beak of small size, which is angular on the back but not 

 incurved. Area moderately large, and perforated by a large circular fora- 

 men and broad triangular deltidium, which is partially closed by a small 

 deltidial plate on each side. Dorsal valve flattened or shghtly convex, 

 often with a broad undefined sinus in the middle, which may be occupied 

 by a single strong plication or by three or five smaller ones. Surface of 

 the shell marked by numerous strong concentric lines of growth crossing 

 the plications, and the minute structure of the shell beautifully and strongly 

 punctate, the punctae being arranged in quincunx order, so as to form oblique 

 lines across the plications. In the interior of the ventral valve the cardinal 

 muscles have left a pair of strong flabellate scars occupying the upper third 

 of the valve, and in the dorsal the adductors show deep semi-circular pits 

 of moderate size separated b}^ an elevated mesial septum which extends to 

 some distance beyond the middle of the valve. The teeth- sockets are large 

 and deep, and the cardinal process double or in some cases triple, nearly 

 erect, and of large size. The crura start from a more or less well-devel- 

 oped hinge-plate at a moderate distance from each other, and are directed 

 forward and outward, giving origin to crural process at a short distance 

 from the hinge-plate. The loop is formed of a rather broad band or rib- 

 bon, which curves outward from the crural points and forms an imperfect 

 circle of nearly one-half the diameter of the valve, and of equal length, 

 the two sides nearly meeting in front, where they are bent upon themselves 

 and, returning at a short distance above the main portions, are united in a 

 transverse bar just in front of the crural points. The main portions of the 

 loop are united to the central septum of the dorsal valve by a transverse 

 plate near the end of the septum and just in front of the middle of the 

 loop, thus giving a double attachment to the loop, as in the living forms of 

 the genus. Among several specimens examined showing the loop more or 

 less perfect, the ventral side of the band comes so near to the ventral valve 



