PLEISTOCENE PECTENS. 127 



anterior ear similar to that of the right valve, except that it is more strong-ly 

 radially ribbed and is less sinuated; posterior ear similar to that of right valve, 

 except that it is more strongly ribbed. Interior of left valve, similar to that of 

 the right, and having the radial ribs developed only after attaining an altitude 

 of about 15 millimeters. 



Dimensions. — Alt. 17 mm.; long. IS mm.; hinge line 9 mm.; diameter 3..") nuu. 



This species is verj- closely allied to P. alasl-ensis, of which it is undoubtedly 

 the precursor. It differs from the latter in having the external ribs on the left 

 valve more prominently im))ricated aliovc, and in having the internal ribs of both 

 valves appearing onlj' after an altitude of about 15 millimeters has been attained. 



The relations existing between P. riversi and P. nlaskensis are a good 

 illustration of H3'att"s law of acceleration of development." In the former, the 

 Pliocene precursor of the latter, the internal riblets are developed only in the 

 adults or after an altitude of about 15 millimeters is reached. In /-". itlaskensis, 

 the living species, this characteristic has l)ecn pushed l)ackward in the process of 

 development to or nearly to the emlu'yonic stage, continuing, howevei', in the 

 later stages of growth, and appearing as a well-developed internal riblet fiom 

 near the umbo to the periphery. 



P. rifersi is known at pi'esent from onl\' two localities, the Pliocene of Santa 

 Monica Canyon and the Pleistocene (lower San Pedro formation) of Deadman Island, 

 near San Pedro. The species is named in honor of Dr. J. J. Rivers, of 

 Oceanpark, whose work in the fossiliferous dei^osits of Santa ^Monica and vicinity 

 has thrown much light on the distribution of the later Tertiary and Pleistocene 

 formations. 



The type, which is now in tiie collection of Delos Arnold, is from the 

 Pliocene of Santa Monica Canyon, Los Angeles County, and was collected by 

 Doctor Rivers. 



Pleistocene (lower San Pedro formation), Deadman Island, near San Pedro (Arnold). 

 Pliocene. Santa Monica Canyon, Los Angeles County (Rivers). 



PLEISTOCEXE I'ECTEXS. 



[Those known lii-st t<> uppear in the Pleistocene formations.] 



Pectex (Pecten) niEGENSis Dall. 

 PI. L], tigs. 1, I<(, and \h. 



1844. Pecten florklKS Hinds, Zool. Voy. Snlpli., 1844, p. 60, pi. 17, lig. 6; not (Mrai.=(Perleii) Jlotida 



Gmelin, 1792 (fide Dall). 

 1852. Pecten floridus Hinds, Reeve, Conch. Icon., vol. 8, 18,52, pi. 8, fig. and sp. ;-i4. 

 1888. Pecten {Vola) floridiis Hinds, Kiister and Kobelt, Conch. -Cab., vol. 17, pt. 2, Spondylus und 



Pecten, sp. 173, taf. 57, fig. 2. 



rt " All modifieations and variations in progressive series tend to appear first in the adolescent or adult stages of 

 growth, ana ilifii to be inherited at earlier and earlier stages, according to the laws of acceleration, until they either 

 become embryonic- or are crowded out of the organization and replaced in the development by characters of later 

 origin".— A. Hyatt, m " Genesis of the Arietidffi," preface, p. ix. 



