PALEONTOLOGY ZORRITOS FORMATION 107 



elongated rhomboidal shape, inequilateral. Base elliptical. 

 Anterior end rounded, the margin sloping posteriorly in an 

 even curve from the hinge-line, with which it forms approx- 

 imately a right angle. Posterior margin produced beyond 

 end of hinge-line, with which it makes an incident angle of 

 about 133. Beaks not prominent, situated near anterior 

 third of length, sulcate near apex, the ribs anastomosing 

 into the sulcus, and the sulcus dying out entirely on the main 

 slope of the shell. Umbonal slope widens rapidly from apex 

 of beaks, curving in even contour anteriorly and forming a 

 perceptible keel posteriorly. Ribs, 23 in number, not sulcate, 

 square in shape over the entire length of the shell, with 

 terior wing and extreme anterior end. Ribs show growth- 

 interspaces wider than the ribs mesially, vice versa on pos- 

 lines, which in places tend to form nodes, but no distinct or 

 regular quadrisection is evident to the naked eye. Micro- 

 scopic examination reveals an irregular but distinct nodosity. 

 Cardinal area low and narrow, with elevated margin behind 

 the beaks, and with two 1 wave-like furrows. Teeth small, 

 fine, vertical. Inner margin fluted. Length, 16; altitude, 

 10.25 >" semi-diameter, 5 ; length hinge, 12.5 mm. 



This small and delicate Area is represented in the collec- 

 tion by one left valve and a young incomplete right valve. In 

 general characters it is closely similar to A. cibaoica 

 Maury 106 from the lower Miocene of Santo Domingo, but it 

 lacks the marked submedian sinus of cibaoica, has decidedly 

 less elevated beaks, and is less produced anteriorly. The 

 umbonal sinus of vanholsti may represent the incipient devel- 

 opment of a more general and pronounced undulation such 

 as that of cibaoica. In vanholsti the sinus, though sharp, is 

 not prominently noticeable, since it is small, confined to the 

 upper surface of the beak, and does not extend to the lateral 

 surface of the shell. 



106 Bull. Amer. Pal., vol. 5, No. 29, p. 173, pi. 30, figs. 19, 20, 

 1917. 



