DESCRIPTIONS OF UPPER JURA FOSSILS — PELECYPODA. 429 



The longest specimen collected by Curtice measured 16 centimeters, 

 and Avhen complete was about 20 to 22 centimeters; the diameter at 

 about one-half the length of the alveolus was estimated on account of 

 the position of the cast in the rock, and this was between 13 and 15 

 millimeters in its longest diameter, and, as actually measured, 10 milli- 

 meters in the shortest diameters. At the distance of 16 centimeters the 

 guard was about 7 millimeters in longest diameter. 



The alveoli are mere casts of the free ends of the shells, but they are 

 universally short, rapidly tapering and much compressed. The largest 

 specimen collected by Dr Curtice was 17 millimeters in diameter near 

 the end of the compressed alveolus. This was a rotund ellipse, and 

 probably this was the real form of the guard. 



The writer visited the localit}^ in company with Dr Curtice during the 

 summer of 1892. The fossilization of these Belemnites is very peculiar, 

 and in imperfect specimens, of which there are considerable numbers, 

 very misleading. The casts of the guards are divided by irregular septa 

 of oxide of iron, giving the ordinary imperfect hollow moulds the aspect 

 of a plant fossil or a very poorly preserved Orthoceratite with distorted 

 septa. This is probably due to the movement of the rocks, which 

 fractured the straight guards and allowed the water to penetrate the 

 fissure and deposit a layer of iron ; then when the shell was wholly 

 dissolved the hollow cast presented the aspect of a tube divided by 

 irregular septa of oxide of iron. Long cylindrical fossils found at Chili 

 bar, also in black slates, exhibited more or less regular cracks of this 

 kind not unlike the sutures of Orthoceratites, and are probably fossils of 

 this species of Belemnites. 



PELECYPODA. 



Avicula, sp. (?) 

 Loc, Stanislaus river, as above. 



The most perfect specimens closely resemble the so-called Aucella im- 

 pressx, Quenstedt, from the Upper Jura, but this shell has a small Aucella- 

 like ear in the right valve, and I could not tell whether what I saw in 

 this specimen is a similar ear or a tooth. Quenstedt's shell is, as pointed 

 out by Lahusen, not a true Aucella. I have referred this species for the 

 present to Avicula, because there are characteristics in both directions. 



There are fragments of several species of bivalves and of Pectens from 

 the strata of Stanislaus River locality that would be very useful if more 

 perfect. 



Associated with these there is a Tarbo (?) and a species of Cerithium, 

 which are not of much value for the definition of the age of the rocks in 

 which thev occur. 



