136 



University of California Publications in Geology [Vol. 11 



This species has been listed in several papers as having a range 

 from Lower Miocene to Upper Miocene. The writer doubts very 

 much whether it occurs above the Oligocene. 



Family Periplomidae 



Genus PERIPLOMA Schumacher 



PEEIPLOMA UNDULATA, n. sp. 

 Plate 15, figures 3 and 4 



Type specimen 11141, Coll. Invert. Palae. Univ. Calif., loc. 1309 

 Shell elliptical, thin, inequivalves ; beaks in the posterior third of the shell 

 strongly inturned and opisthogyrous. Anterior end broadly and regularly 

 rounded ; posterior end subtruncate, the margin being almost at right angles 

 to the posterior ventral edge. Anterior dorsal margin long, nearly straight ; 

 posterior dorsal margin short and slightly concave. Surface covered by irregular 

 concentric undulations, on and between which are the finer incremental lines. On 

 the type, a right valve, these undulations are rather conspicuous; on other speci- 

 mens they do not show so prominently ; valves also finely striated, the striations 

 being internal and showing only on eroded specimens. Right valve with posterior 

 dorsal margin strongly depressed; just anterior to the depressed area there is a 

 somewhat obscure, shallow sulcus, which extends from the beak to the ventral side 

 of the posterior end. On the left valve the depression along the posterior margin 

 is not so strong, and there is no sulcus corresponding to that seen on the right 

 valve. 



Dimensions. — Type specimen: length, 27 mm.; height, 15.5 mm. 

 Occurrence. — University of California localities 1309 and 1131. Type from 

 locality 1309. 



P. undulata is closely allied to P. argentaria Conrad, 113 a Recent 

 West Coast species. The two are very similar in outline and the 

 depressed posterior dorsal area of both valves are almost exactly alike. 

 The most important differences between the two species are as fol- 

 lows: Thi- posterior end of P. argentaria is rounded, while on P. 

 undulata it is subtruncate; the surface of P. argentaria lacks the 

 irregular undulations described above. The hinge plate of a right 

 valve of P. undulata was exposed; this shows the chondrophore to be 

 deeper than that of P. argentaria, the axis being almost parallel with 

 the anterior dorsal margin, while on the latter the axis has a consider- 

 able angle between it and the posterior dorsal margin. 



us For original description of P. argentaria Conrad, see Jour. Phila. Acad. 

 Sci., vol. 7, p. 238, pi. 18, fig. 8, 1837. P. planiscula Sowerby is listed by Arnold 

 as a synonym of P. argentaria. The former name is given by Keep in his "West 

 Coast shells," 1911 edition, as having precedence; figure 84 on p. 108 by Keep, 

 shows the interior of this species. 



