MIOCENE PECTENS. 85 



of California from Cuaymas, M(?xico. At the type locality it is associated with 

 the following fauna: Pecteii (jarrisoensis, Pcvten l'i>ej)i, Curdlam sp.. Cluom- n. sp., 

 Glycymeris sp., Ostrea sp. (large), Codahla sp. (very large), Seint'l,-. {aff.) publini, 

 Cardlta {Car'diiaincra) sp., Strombi/x sp., Mali'ct (atf.) r/'»(/>-ii.'<. echinoid spines, etc. 

 The type of P. var. ■mendeiihnlll is U. S. N. M. No. l<;-i84!», and is from Santa 

 Rosalia, Lower California, where it was collected by Doctor Merrill. Named in 

 honor of Mr. Walter C. Mendenhall. of the Ignited States (Jeological Survey. 



RANGE. 



Miocene. Santa Rosalia, Lower California (U. P. Merrill; K. Taliner); head of 

 Garnet Canyon, about 12 miles north of the Mexican boundary, and Alverson 

 Canyon, about S miles north of the Mexican boundai-y. San Diego tbunty 

 (W. C. Mendeniiall: S. l?owers). 



Pecten (Plagioctenium) deserti Conrad. 



PI. XXVI, figs. 1, 2, 'In, .S, 4, and ia. 



1855. Feclen deserti Conrad, Desc. Fos. and Shells., House Doc, 129, .July, 1S.55, p. 15 (HiU nail). 

 18-56. Pecten deserti Conra<l, Pac. R. R. Rept., vol. -5, 1856, p. 325, pi. 5, fig. 41. 

 1888. Pecten deserti Conrad, Cooper, Seventh Ann. Rept. Cal. St. Min., for 1887, p. 257. 

 1894. Pecten (Plagioctenium) deserti Conrad, Dall, Trans. Wagner Free Inst. Sci., vol. 3, pt. 4, 1S!)8, 

 p. 703. 



/Mscr/'/jf/o/t. — Shell averaging about o'J millimeters in altitude, aljout as long as 

 high, biconvex. e((uivalve, subequilateral, and witii serrate margins; sides straight 

 and sloping at a steep angle. Right valve with '22 or 23 high, narrow, close-set, 

 smooth, rounded ribs, whi(di sometimes show looped incremental lines on their 

 tops near extremities; interspaces deep, narrow, slightly concave bottomed, and 

 ornamented on the bottom by numerous tine, sharp, straight, incremental lines; 

 hinge line about three-fourths length of disk; ears subeipial; the anterior slightly 

 convex and ornamented by 4 radiating ridges and numerous incremental lines, 

 while the posterior is ornamented by 6 or 7 radials and numerous concentric 

 lines; byssal notch rather small. Left valve similar to the right, except possibly 

 that the ri})s are relatively slightly taller. Hinge strong, with a deep resilial pit 

 and a submarginal channel parallel with the upper margin; luunerous parallel 

 microscopic lines perpendicular to the hinge line visible in the crural grooves. 



Duitensions. — Alt. 32 mm.; long. .33mm.; hinge line 2.5 mm.; diameter 17 mm. 



This species is distinguishable from allied forms by its strong hinge, tall, 

 narrow, close-set ridges and straight incremental sculpture in the bottoms of the 

 intei'spaces. 



P. deserti has so far been found only in beds supposed to be of Miocene age, 

 in the Carrizo Creek district, Colorado Desert, San Diego County. From the 

 evidence at hand it seems likely that P. deserti occupies a somewhat difierent 

 horizon (possibly lower) than that containing /'. carrizoenHif^ P. Iceepi, and /'. 

 cerrosensis var. niendenhalU. 



RANUE. 



Miocene. East end of Carrizo Mountain, and Ulsa oil well, north of the Mexican 

 boundary, San Diego County (Conrad; Fairbanks; Bowei's; Mendenhall). 



