90 



University of California Publications. 



[Geology 



the described species, and a description of the forms recognized 

 by him, together with memoranda of their occurrence and the 

 associated fauna. 



In this work the writer has had access to the collections of 

 the University of California, Stanford University, the California 

 Academy of Sciences, and the California State Mining Bureau. 



arc A microdonta, Conrad. 

 Plate 8. 



Area microdonta. Conrad, 1853-4. Pac. E. E. Reps., Vol. V, p. 323, 

 PI. Ill, fig. 29. 



Description. — This species was originally described by Conrad 

 as follows: "Rhomboidal, ventrieose, thick in substance, anterior 

 side very short, umbonal slope rounded. Ribs 25, prominent, 

 narrow, wider posteriorly except on posterior slope, where they 

 are small, and not prominent, about five in number. Cardinal 

 teeth small, numerous, closely arranged, margin profoundly 

 dentate, dorsal area rather wide and marked with about six im- 

 pressed lines, beaks distant. 



The variations in this form, as shown in figs, la to 36, is con- 

 siderable, and was at first thought to be of specific value. The 

 specimen shown in figs. 2, 2a, and 26 was, however, found to 

 bridge over the differences and lie near Conrad's type. It was ac- 

 cordingly figured as the best representative of the type, along' with 

 outline representations of other forms in figs, la, lb, 3a, and 36 

 for comparison. The specimen shown in figs. 3a and 36 is rather 

 small, measuring 45x33 mm. It is more ventrieose and more ine- 

 quilateral than A. trilineata. It has 27 somewhat beveled ribs, 

 showing no sign of a median groove. The shell is wide and heavy, 

 with high beaks separated by a profound ligament area. The 

 posterior end is noticeably concave as far forward as the fifth or 

 sixth rib, as shown in Conrad's figure. 



The individual represented in figs, la and 16 is very inequilat- 

 eral, extremely ventrieose, with beaks very heavy and high, and 

 the ligament area flaring and enormously developed, making- the 

 thickness of the closed shell fully as great as its height. The 

 hinge line is relatively short, the beaks strongly incurved, 

 though very distant, and placed so close to the anterior end as 



