PLACIPHORELLA. 307 



Interior white, slightly blue tinted. Sinus in valves i to vi repre- 

 sented by a very shallow wave, the sutural plates continuous, being 

 connected by aplate which fills the sinus like a key-stone ; in valves 

 vii to viii the sinus is deeper and more distinctly angular. Anterior 

 valve having 8 slits, the teeth obtuse, short, often bilobed or con- 

 spicuously rugose. Central valves having 1 slit, teeth wedge-shaped, 

 thicker at the edges of the slit. Posterior valve having a very 

 heavy callus supporting the short, rugose insertion-plate, which is 

 interrupted posteriorly by a broad, shallow sinus, and has 1 slit on 

 each side (occasionally two on one side). 



Girdle very broad in front, reddish, fading to yellow toward the 

 outer edge, irregularly and sparsely beset with scaly hairs (fig. 13), 

 of which one or two are generally to be seen in each suture ; a close 

 fringe of short (broken) hairs adorns the girdle-edge. 



Length 50, breadth 38 mill. ; divergence 135. 



Length 30, breadth 24 mill. ; divergence 130. 



Humboldt Bay (Newcoinb) and Monterey Bay (Canfield, Hemp- 

 hill, et. al.), California ; Todos Santos Bay, Lower California (Hemp- 

 hill). 



Placiphorella velata CPR., MS. DALL, Proc. U. S. Nat. Mus. 

 1878, p. 303, 307 (description of animal) ; p. 298, t. 2, f. 36, a 

 (dentition). 



This species is distinguished from the the next by its differently 

 formed posterior valve, by the connection of the sutural plates 

 across the sinus, and the anterior projection of the tegmentum at 

 the sinus. The largest specimens I have seen were collected by 

 Hemphill at Todos Santos Bay. 



Fig. 12 represents the insertion-plates of an intermediate valve 

 seen from their summits, showing the subtriangular form of the 

 posterior insertion-tooth, the non-thickened edges of the slit, etc. 



P. STIMPSONII Gould. PL 62, figs. 84, 85, 86, 87. 



Shell broadly oval, depressed, subangular, with straight side- 

 slopes. Color whitish along the middle, the sides mottled and 

 streaked with greenish-yellow, olive and blue, the colors so blended 

 as to give a general effect of dark olivaceous. 



Valves not beaked, sculptured with unequal growth-lines. 

 Lateral areas having a moderately prominent diagonal rib, with an 

 inconspicuous wider, lower, sutural rib, the space between excavated. 

 Anterior valve narrowly crescentic, concentrically striated, but 



