DISCUSSIONS OF THE SPECIES 



CRANIA SILURIANA, Hall, 1863 



PLATE I, FIGS, i, 2 



, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist, p. 148, pi. 21, figs. 3-7. 



1879- 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 282, pi. 21, figs. 3-7. 1882. 



But a single embryo of this species has been found in all the material 

 examined. It is of an incipient stage of growth, measuring but i mm. 

 in height, and 1.5 mm. across the aperture. Compared with the mature 

 form, the average size of which is about 9x20 mm., it shows a relatively 

 greater elevation and a more regularly conical form. Otherwise, all the 

 few essential characters of the adult shell are present at this early age. 



ORTHIS ELEGANTULA, Dalman, 1827 



PLATE I, FIGS. 3-11 



, Hall. Twenty-eighth Rept. N. Y. State Mus. Nat. Hist., p. 150, pi. 21, figs. 11-17. l8 79- 



, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 285, pi. 21, figs. 11-17. 1882. 



Both the species of Ortliis occurring at Waldron (O. elegantula and 

 O. hybrida) are very abundant. In the later stages of growth, the former 

 species is readily distinguished from the latter by its flatter and shallower 

 dorsal valve and deeper ventral valve, features which usually hold good for 

 purposes of discrimination ; but in extremely early stages of growth, the 

 nearly equivalve form of the shell makes the separation of the species very 



