46 



TERTIARY AND QUATERNARY PECTf^NS OF CALIFORNIA. 



uinbonal region. These pits may be shallow or deep; their liasal margin sometimes 

 projects slightly into the t-avity of the valves; their apex is always nearly coincident 

 with the umbonal point of the \'alve. In a few species in the right valve the lateral 

 margins of the pit are raised into tooth-like proces.ses, which tit into corresponding 

 depressions in the opposite valve (e. g., P. (suuftti Bernh.). but these are not 

 homologous with the so-called teeth of Plicatuhi and SpondijluK. Outside of the.se, 

 radiating fanlike from the apex of the valve, are frequentl}' found one to three 

 pairs of more or less prominent laniina>, which 1 call the cardinal crura, and farther 

 away and below on the ridges which mark the lower boundary of the eai's will 

 sometimes be found another pair, only distally conspicuous, which 1 have named the 

 auricular crura. The cardinal crura are most conspii-uous in heavy shells, especially 

 such as Pecit'ii proper and Lyrnpecten. and serve to adjust the closing of the valves, 

 as does the hinge armature of the Teleodonts. In a few species the crura are 





Fig. '1. — Same -species as fig. 1. Right valve; interior. A, 1'. B and U as in tig. 1: W, hinge line; rp, resilial pit; j>7J, 

 provinculum; cc, c.r, cardinal crura; hn, byssal notch; cm, ctenolinm; as. adductor scar. 



sufficiently prominent to actually interlock with the valves half open; in many others 

 hardly any trace of them is visible. Alnio.st all species pos.sess in the nepionic stage 

 a well-marked provinculum, formed l)y an elongated area on each side of the pit, 

 covered by long, narrow, close-set taxodont teeth, separated by narrow grooved 

 interspaces. In most species the provinculum is evanescent or represented in the 

 adult only by faint vertical striae, which cross the cardinal crura. In a few small, 

 thin-.shelled, mostly deep-water species the provinculum is persistent and functional 

 (e. g. , P. thalassi.nutii Dall), forming an interlocking hinge. In Pecten proper, 

 Ohlamyn, and some other groups the upper cardinal margin of the right valve is 

 bent over that of the left valve. There are occasional species in which the adult 

 valves have each a flat area along the whole cardinal margin, covered by the ligament 

 and forming a V-shaped groove between the upper margins of the valves, as in P. 

 s^viftii. The disk of the valve is usually rounded or oblique below and at the sides, 



