405 



Micro. 



Uc. 1 



Microdiscus punctatus, (Salter, Q. J. G. S. London, 1864, 



XX, 237, pi. 13, fig. 11— Whiteaves, 

 Am. J. Sc. 1878, xvi, 'i^'^b,— Micro- 

 discus pulchellus^ Hartt, No. 13 of 

 J list sent to Dawson.) Walcott, Bull. 

 U. S. G. S. No. 10, page 24, plate 2, 

 figs. 1, la, 15, head shields (gla- 

 bellas) showing variations of form 

 and making, enlarged four times. 

 Fig. 1<?, tail piece (pygidium) en- 

 larged three times. — Middle Cam- 

 hrian {Saint John) formation, New 

 Brunswick, and New Foundland. — M. C. 



Microdiscus quadricostatus, (Properly a young Irinu- 



cleus.) Emmons, American Geology, Vol. 

 1, part 2, page 116, plate 1, fig. 8, enlarged 

 ahout five diameters, Wolcott, in Bull. U. 

 S. G. Sur. No. 30, page 152, says, Emmons' 

 genus Microdiscus was founded on a speci- 

 men of Trhiucleus. — Barrande thought 

 Emmons' minute forms might be the young 

 fry of some large trilobite like Trinucleus. (Salter.) — Now, 

 many of these minute species are known {jpunctatus^ speciosus, 

 etc.,) and grouped as Microdiscus^ midway between the Agnos- 

 ^^^5 and the Conophrys groups. Pemphigaspis iullata (Hall, 

 16th An. Rt. p. 221) is closely related. — Emmons' specimens 

 were found in the White fragile [H. River) shales. His 

 name Microdiscus cannot be applied to his specimens ; but it 

 is retained for all the minute Cambrian species. (Walcott.) 



Microdiscus speciosus, (Ford. 1873, Am. J. S. VI, p. 137, 



fig. 2 a, I. XIII p. 141.) Wal- 

 cott, Bulletin U. S. G. S. No. 30, 

 ^ page 154, plate 16, fig. 3, Za^ top 

 I and side, enlarged twice ; 3b, 

 tail (pygidium) enlarged twice. 

 Fig. 3c. very perfect head from 

 Troy. — Lower Camlrian {Geor- 

 gian) formation in Canada, and 

 not rare near Troy, N. Y. — 



