6 



( 'umings — Morphogenesis of Platystrophia. 



like that of the nepionic Platystrophia (fig. la') — a ventral 

 valve in both cases. 0. lyncioides also has a strong median 

 plication and about four weaker ones on either side. The pos- 

 terior half of the valve is devoid of longitudinal markings, but 

 possesses concentric lines. The beak is well elevated, though 

 the area is not exposed in any of Dr. Matthew's specimens. 

 As shown in Davidson's figures of 0. lenticularis the area is 

 high and the foramen large and triangular.* Our Ho;, lc indicates 

 that the ventral muscular impression is about the same as in 

 other primitive Orthidce (Platystrophia, Plectorthis, etc.). 



The name which Dr. Matthew has given this pretty little 

 variety of Orthis lenticularis indicates, as he also expressly 

 states, that he considered it as a possible ancestor of Platystro- 

 phia. My own conclusion to the same effect was reached 

 independently through a comparison of the nepionic Platy- 



Fig 2. a, a', a", a'", posterior, ventral, profile and dorsal outline views of 

 the nepionic Platystrophia ; actual size shown by small figures to the right. 

 b, b', ventral and dorsal views of Orthis Hicksii, after Davidson, c, c', ven- 

 tral and dorsal views of Orthis salemensis, after Walcott. 



strophia with Davidson's figures of 0. lenticularis. Aside 

 from the strong resemblances pointed out above, the wide dis- 

 tribution, great abundance, and high variability of this species 

 make it an ideal ancestral type. It may very well have pro- 

 duced, on the one hand, the pauciplicate group, Platystrophia^ 

 * Silurian Brachiopoda, 1869, pi. xxxiii, fig. 26a. 



