1896. | NEW YORK ACADEMY OF SCIENCES. 209 
joint of the rachis has no protecting cover, except in so far 
as the attenuated posterior lobe (basal lobes) of the glabella 
“may assume that office, an unusual condition with trilobites. 
A. regulus is the only species known to me that has an occipital 
ring. 
Agnostus is the sole genus among the early Cambrian trilo- 
bites, except Microdiscus, in which the pleural groove runs for- 
ward in going outward toward the extremity of the pleura, and 
in association with this we find the point of the pleura in most 
Agnosti turned forward,* this is especially plain in Longi- 
frontes. In Fallaces the pleural groove on the posterior seg- 
ment lies along the front of the pleura, but on the anterior 
segment at the back; hence the pleura in this group is inflated 
in the middle and tubercle-like; on the anterior segment there 
are really two grooves, of which the anterior affords an inter- 
locking edge with the marginal fold of the head shield when the 
body if folded together. 
The long pygidium of an Agnostus assures us that this part 
has undergone very considerable changes from the early larval 
moults, but the proof is not often apparent; an examination of 
the tests of two species (A. Acadicus var. declivis and A. 
Nathorstt) give an inkling of the way in which the changes have 
ome about—those which resulted in the three-lobed rachis of the 
early Agnosti. 
In A. Nathorsti and other Lonigifrontes the attenuated and de- 
pressed end of the rachis is the original pygidium, the front of 
this part is marked by a minute tubercle (to be found only in 
well preserved tests); in a rare example of the pygidium of de- 
clivis three pairs of scars behind this tubercle point out the 
existence of three somites here; two pairs of scars in front of 
this little tubercle indicate the presence of two more somites, 
which complete the posterior lobe of the rachis; these two som- 
ites swell out to greater width and height than those behind ; 
the anterior of these two somites is also sometimes further 
marked off by a pair of furrows, one on each side of the rachis 
corresponding to the oblique furrow on each side of the anterior 
lobe of the adult rachis; in Regii these furrows are so strongly 
developed that this somite was counted by Barrande as a part 
of the middle lobe of the rachis. The middle lobe and the 
anterior lobe of the rachis form another enlarged pair of 
somites with a tubercle or spine at the back, and correspond to 
the two front somites of the posterior lobe. 
We thus see that by its pygidium Agnostus shows three stages 
*An exception to this almost universal rule is A. granulatus Barr., but it is one of 
the few Agnosti that have genal spines. 
NotTe—204 of last line p. 208, should be 210. 
TRANSACTIONS N. Y. ACAD. SCI., XY., Sig. 11, August 14, 1896. 
