Clark: The San Lorenzo Series of Middle California 127 



Family ARCIDAE 

 Genus ARCA Lamarck 



AECA (SCAPHAECA) MEDIAIMPEESSA, n. sp. 

 Plate 7, figures 7 and 8; plate 16, figures 5, 6 and 7 

 Type specimen 11174, Coll. Invert. Palae. Univ. Calif., loe. 1131 



Shell small to medium in size, rhombic in outline. Umbones rather broad, 

 strongly inturned and anterior to middle of shell, depressed medially, the depres- 

 sion on the smaller specimens extending from the beak well down to the ventral 

 edge; this character is best seen on the smaller specimens, being less noticeable 

 on the larger ones. Posterior end broadly subtruncate to gently rounded , 

 anterior end evenly rounded, narrower than posterior end. Surface sculptured 

 by twenty-seven to thirty flat-topped, radial ribs with interspaces about as wide 

 as the ribs. On one of the specimens figured (pi. 7, fig. 7, specimen 11172) 

 most of the ribs show a medium groove beginning about half way from the beaks 

 and extending to the ventral edge ; this groove is similar to that seen on Area 

 montereyana Osmont and on Area triUneata Conrad. 100 On this last, there is 

 often a second and third groove bordering on each side of the medium groove 

 which begin lower down from the beaks than does the medium groove. It is 

 very possible that if larger specimens of Area mediaimpressa are found, the 

 second and third grooves may also appear on the lower part of the ribs. Appar- 

 ently the ribs were originally somewhat spinose, due to the sharp, imbricating, 

 incremental lines ; this is to be seen on a few of the better preserved specimens. 

 Surface rather strongly depressed posteriorly. Cardinal area very narrow ; 

 ligamental grooves obscure. Hinge plate narrow, slightly curved, with about 

 thirty-five taxodont cardinals, which, beneath the beaks, are very small ; distally 

 they become larger and converge ventrally. 



Dimensions. — Type specimen : length, 14.5 mm. ; height, 6 m.m. ; length of 

 hinge plate, 6 mm. ; length of hinge plate anterior to the beak, 3.5 mm. 



Occurrence. — University of California locality 1131. 



Area mediaimpressa differs from the typical A. montereyana, 

 found in the Area montereyana beds of Contra Costa County, in 

 the following respects: The beaks are more prominent and broader, 

 the latter lacking the depressed area just below the beaks, and the 

 truncated posterior margin being more oblique; A. montereyana 101 

 is the longer and is more strongly produced posteriorly, also the 

 surface is not so strongly depressed near the posterior truncated end. 



Osmont seems to have included two distinct species in his descrip- 

 tion of Area montereyana; the type, which is figure 5 on his plate, 

 undoubtedly came from the Area montereyana beds of Contra Costa 



100 For original description of Area trilineata see Pacific Bailroad Rep., vol. 6, 

 p. 70, pi. 2, fig. 9. 1854-1855; for good redescription see Osmont, Univ. Calif. 

 Publ., Bull. Dept. Geol., vol. 4, no. 4, p. 91, pi. 9, figs. 4~4e, 1904. 



101 For original description of Area montereyana see Osmont, V. C, Areas of 

 the California Neocene, Univ. Calif. Publ., Bull. Dept. Geol., vol. 4, no. 4, p. 96. 

 pi. 9, fig. 5 (not 5a and 56), 1904. 



