ON THE DEVELOPMENT OF THE SHELL OF ZYGOS- 

 PIRA RECURVIROSTRA. 



BY CHARLES SCHUCHERT. 



The material showing the ontogeny of the shell in Z. recurr/- 

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

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

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

 brachiopods are Orthis testudinaria, 0. meedsi, Pledambonites 

 xovYva, Strophomena scofieldi, Clitatnbomtes diversa, etc. 



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

 of .8 mm., and is elongate subtriangular in outline, biconvex, 

 with the ventral valve a little the deeper ; ventral beak acumi- 

 nate, inclined posteriorly at an angle of about 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 delthyrium, but 

 apparently not attached to the rostral cavity. The fold and 

 sinus are faintly developed, 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 by three short plica- 

 tions and the ventral fold by two. In specimens of a somew r hat 

 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- 

 pare 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, their 

 number being increased as growth proceeds by the addition of 

 others along the lateral margins of both valves. The plications 

 remain simple throughout. 



The smooth nepiastic stage gradually grows more and more 

 rotund and subquadrangular in outline, and at maturity is pli- 

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

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

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



11 BIOL. Soc. WASH., VOL. VIII, 1893. (79) 



