MIOCENE PECTENS. 75 



1888. Liropecfen eMrdlanus Conrad, Cooper, Seventh Ann. Rept. Cal. St. Min., 1888, p. 246. 



1894. Llropeeten, entrellanuK Conrad, Coojwr, Bull. Cal. St. 'Slin. Bureau, No. 4, pt. 5, 1894, p. .57, i)l. .5. 



figs. 65-67 (te.xt only in part). 

 1898. PecWn {Lyropecten) Heermauni Dall, Trans. Wagner Free Inst. Sd., vol. 'A, pt. 4, 189S, p. 701 



(not P. fleennanni Conrad, Proc. Acad. Nat. Sci. Phila., vol. 7, I8.">5, [i. 267, which is P 



caurhms Gould). 



Description. — Shell averagino- aboiit 1*0 inillimeter.s in altitude, somewhat 

 longer than high, equivalve, equilateral, of medium thickness, generallj- ([uite 

 ventricose, and with margin.? more or less .serrate; base evenly rounded; sides 

 sloping, slightly concave above. Right valve with 16 to 20 prominent, squarish 

 ribs separated by channeled interspaces which are narrower than the ribs; inter- 

 spaces each with one sharply outlined, squarish riblet in the middle; surface 

 sculptured by tine raised incremental lirula? which loop back over the ribs and 

 riblets, and, on the anterior and posterior portions of the disk, by radiating striie, 

 which increase in number as the periphery of the disk i.s approached; hinge line 

 [straight and nearly equal to one-half length of disk; ears eqtial; anterior ear 

 arcuate in front and ornamented above byssal area b}' three or four radiating 

 ridges and numerous incremental lines; by,ssal notch sharplj' defined; bys.sal area 

 with incremental sculpture only; posterior ear with slightly concave end and orna- 

 mented by 5 or 6 radiating ridges and more or less prominent incremental lines. 

 Left valve with the ribs narrower and having more sloping sides and rounded 

 tops than those of the right; interspaces wider; auxiliary riblets inclined to be 

 dichotomous near the ends in the more mature specimens; the looping incre- 

 mental lirulaj and the secondarj' radiating lines apparently more highly developed 

 in this valve than in the right; ears similar to the posterior ear of the right valve. 



Dimensions. — Alt. 93 mm.; long. 105 mm.; hinge line 50 mm.; diameter 47 mm. 



This is a mo.st variable species in size, shape, and sculpture, the above 

 description applying to the most common form. One peculiar characteristic 

 .sometimes shown bv this species is a contraction or interruption in the growth at 

 diflerent stages. See PI. XXI, figs. 1, la, Vj. Up to the first interruption the 

 sculpture is generally simple and quite constant, but afterwards it (especially the 

 radial) becomes more complex, the riblets in the interspaces becoming dichotomous 

 and the radiating lines becoming more numerous and prominent. 



The variety of this species described as Lyropecten volmformis by Conrad is the 

 extremely convex type. Conrad thought that the valves were unequal in the very 

 convex shell, but in all the specimens of this sort examined by the writer, both 

 valves show an equal amount of convexity. This very ventricose form is common 

 near Coalinga, Fresno County, in beds of probable San Pablo (upper Miocene) age. 



Specimens of this species from Catalina Island show more ribs (right valves, 

 19 to 21) than the average, and also appear to be slightly inequivalve, the left valve 

 being the more ventrico.se. The characteristic interruption and constriction took 

 place on most of the Catalina specimens at an altitude of about 55 millimeters. 



A specimen of this species from San Benito County, collected bj^ Mr. H. W. 

 Turner, is very convex, and shows tteautiful equal, equidistant, looped lamella? over 

 the surface of the valve. The intercalary riblets on this specimen are sharply 

 defined. 



