SILURIAN BRACHIOPODA 31 



The principal differences to be noted in comparison with the adult 

 individuals are the subcircular outline of the shell, the depressed valves, 

 the absence of a median fold, and the large deltidial area. 



ElCHWALDIA RETICULATA, Hall, 1 868 



Plate III, Figs. 11-13 



, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 169, pi. 26, figs. 50-54. 



1879. 

 • — — , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 312, pi. 26, figs. 50-54. 1882. 



Very few of the earlier growth stages of this species have been 

 observed and these show but comparatively little variation from the 

 features of the normal adult. On plate 3 is given a figure of the youngest 

 example found, which has a length and width of 3 mm., while the usual 

 adult is about i6xi6 mm., varying in relative proportions with the increase 

 of senile obesity. The change in outline during growth is from 

 subcircular to subtriangular, and in earlier stages, the ventral fold and 

 sinus are very ill-defined. The peculiar triangular exfoliation of the shell 

 on the umbo of the ventral valve is evidently a constant feature in every 

 stage of growth after the shell becomes attached. The nature of this 

 peculiarity was indicated by Billings in the original diagnosis of the genus 

 (Ann. Rept. Canadian Geol. Survey, 1857-58), and was demonstrated more 

 fully by Professor Hall, in the Twentieth Report on the Condition of the 

 New York State Cabinet of Natural History (pp. 274-278, 1867). This 

 area is underlaid by an internal shelf or diaphragm attached along its lateral 

 margins, and having fully, or rather more than, the width of the median 

 sinus. Through the space thus left between the shell and the internal dia- 

 phragm, communication is afforded with the outside world. Mr. John Young 

 has called attention to the fact that in E. capewelli, the margins of the ex- 

 ternal reticulated layer of the shell about the umbonal bare spot, are rough 

 and ragged, the superficial hexagonal cells being without finish along these 

 edges, suggesting therefrom, that the animal was attached to marine 



