ON THE DEVELOPMENT OF THE SHELL OF ZYGOS- 

 PIRA RECUR VIR OSTRA. 



BY CHARLES SCHUCHERT. 



The material t^liuwing the ontogeny of the shell in Z. recurrl- 

 rostra was gathered from some blue-green shales on St. Anthony 

 hill, a suburb of St. Paul, Minnesota. This horizon is ecpivalent 

 to the lower Trenton of New York. Some of the associated 

 brachiopods are Orthis testudinaria, 0. meedsi, Fledamboii.iies 

 mricea, Strophomend, scofieldi, CUtambonites diversa, etc. 



The youngest specimen observed (plate xi, fig. 1) has a length 

 of .8 mm., and is elongate si»btriangular in outline, biconvex, 

 with the ventral valve a little the deejier ; ventral ])eak acumi- 

 nate, inclined })osteriorly at an angle of al)out 45° to the plane 

 of the valves ; delthyrium triangular, as wide as long and 

 devoid of deltidial plates. In the apical portion there is a short 

 concave plate continuous with the walls of the deltliyrium, Init 

 apparently not attached to the rostral cavity. The fold and 

 sinus are faintly develoj^ed, becoming obsolete at about the center 

 of the shell and are without plications. In other individuals of 

 about the same size the sinus is occupied l)y three short plica- 

 tions and the ventral fold by two. In specimens of a somewhat 

 larger growth these primitive plications are rapidly followed by 

 a number of new ones along the entire anterior margin. The 

 size of the shell at which they begin to develop is variable (com- 

 jtare figs. 2-5), being the earliest in the narrow depressed indi- 

 viduals (fig. 3) and latest in the rounder and more convex 

 specimens (fig. 4). New plications are rarely interpolated, tlieir 

 number being increased as growth proccieds by the addition of 

 others along the lateral margins of both valves. The plications 

 remain simple throughout. 



The smooth nepiastic stage gradually grows niore and more 

 rotund and subquadrangular in outline, and at maturity is })li- 

 cated to the apex of the shell. The ventral beak, which is at 

 first slightly recumljent (fig. la), becomes erect, and finally is 

 strongly incurved over the dorsal umbo (fig. 9a). The large 



11— Bioi,. Soc. Wash., Vol. VIII, 1893. (79) 



