Paleontology Zorritos Formation 115 



an die Seite des Wirbels, was bei den Abbildungen Nelsons 

 unberiicksichtigt ist" in his description of A. larkinii (loc. 

 cit.) makes clear the fact that he was dealing with A. im- 

 porcata, and not the species described by Nelson as larkinii. 

 Other somewhat similar forms in the Zorritos fauna are A. 

 tarocnsis, A. septifera, and A. nelsoni, all of which differ in 

 being larger, heavier forms much different in shape and 

 general contour. Perhaps the most closely similar form out- 

 side of Peru is A. idonea Conrad ^^'^ from the Miocene of 

 Marj'land and neighboring regions. It differs in having 

 squarer ribs, less erect beaks and ordinarily a smaller area. 

 Upper Zorritos: Caleta Grau, Quebradas del Toro, del 

 GriUo. 



Area (Scapharca) obesiformis Grzybowski 



Plate VI — Figs, i, 2. 



Area obesiformis Grzybowski, Neues Jahrb., Beil. Bd. 12, p. 633, 

 pi. 28, figs. 3, 3a, 1899. 



"Schale queroval mit zugestumpften \'order — und ver- 

 langertem Hinterrande. Schlossrand gerade. Area lang, 

 breit, mit schwach angedeuteter Ouerstreifung iiber den 

 dachformigen Furchen. Die hohen Wirbel stehen in 1/3 der 

 Lange und sind 5 mm. von der Mittellinie entfemt. Ober- 

 flache mit 30 starken Rippen und scharf markierten Zu- 

 wachsstreifen. Hohe 30 mm., Lange 37 mm., Dicke (einer 

 Schale) 18 mm."' — Grzybowski, 1899. 



The anterior margin is broadly rounded, meeting the 

 hinge-line at an angle barely obtuse : the ventral margin 

 descends slightly posteriorly ; the posterior margin is pro- 

 duced, curving back to meet the hinge at an angle of about 

 140° — 145.° The umbones vary from nearly erect to decid- 

 edly inclined anteriorly. The ribs, from 28 to 30 in number. 

 are tuberculate anteriorly as in A. larkinii ; the posterior ribs 

 are simple and show only distinct growth-lines. The ribs 



Md. Geol. Surv., Miocene, p. 389. pi. 106. figs, i, 2, 1904. 



