NO. 2297. BRACHIOPOD GENUS PLATYSTROPHIA—McEWAN. 385 



paleontology, United States National Museum, for his suggestions 

 and criticisms in the preparation of the paper. The writer is under 

 deep obligations to Prof. A. W. Grabau of Columbia University, 

 imder whose direction this study was begun. 



GENERIC CHARACTERISTICS. 



Most paleontologists have based their classifications on adult 

 characters alone. If a classification is to express generic relationship 

 and not merely represent a group of morphological equivalents, the 

 early growth stages must not be neglected, and should indeed be 

 regarded with great care. 



The writer was unable to secure young specimens of Platysfropliia . 

 Professor Cumings has, however, demonstrated ^ that a specimen 

 between 0.66 mm. and 1 mm. in length was "markedly transverse, ^ 

 the greatest breadth about one-third of the way from the beaks to 

 the front margin. The posterior margin (cardinal line) is straight, 

 and the anterior semielliptical. The greatest height is at the beaks 

 which project slightly beyond the area, but are not incurved. The 

 area is considerably less than the width of the shell at the hinge, 

 and the large foramen is about equally shared by the two valves. 

 At the apex of the ventral foramen is a small callosity. No deltidium 

 was observed." The young stages are well preserved in many adult 

 specimens and thus furnish a permanent record of the entire life 

 history. These specimens show a smooth shell next to the beak. 

 Very soon, however, plications appear, eight on the brachial valve 

 and nine on the pedicle valve. The furrow separating the two 

 centrally located plications of the brachial valve is deeper than the 

 others and forms a median sinus, and the centrally located plication 

 on the pedical valve is larger than the others and forms a fold. 



This development is followed by the depression of the area occu- 

 pied by the pedicle fold and the elevation of the area occupied by 

 the brachial sinus thus forming the true pedicle sinus and brachial 

 fold. The early nepionic fold becomes a plication occupying a 

 median position in the true sinus and the two plications bordering 

 the early nepionic sinus become plications on the true fold." 



In discussing this development Winchell and Schuchert say, 

 "* * * jj2 several immature individuals it has been observed 

 that in the early neologic stage the beaks are strongly elevated, 

 probably erect, and each has a very large open delthyrium, surface 

 smooth at first, but gradually developing eight plications and a 

 mesial sinus in each valve. The sinus in the dorsal valve is bounded 



' Amer. Joum. Sci., ser. 4, vol. 15, 1903, pp. 2-4. 



2 For a detailed discussion of the early stages of the genus see Cumings, Amer. Journ. Sci., ser. 4, vol. 15, 

 1903, p. 3. 



115690— 19— Proc.N.M.vol.56 25 



