1896. | NEW YORK ACADEMY OF SCIENCES. 215 
The typical form is described as “ Rachis of the pygidium 
long, nearly reaching the margin, lateral lobes greatly narrowed 
behind, scarcely meeting behind the rachis, marginal fold moder- 
ately broad.” 
Size.—Length and breadth of the shield, 3-4 mm. 
There is so much difference in the tests which are referred to 
A. fallax by Swedish geologists that I am constrained to admit 
that the form I have described as A. vir can be included as a 
variety under it, and it is here so described; it is in fact nearer 
the typical form than A. fallax, var. ferox, Tullb. 
Development of the young.—Several small heads and tails 
of the following variety of this species show important differ- 
ences from the adult. The head when of the length of 4 mm. 
shows a narrow, cylindrical glabella, divided into four lobes by 
furrows indenting the sides; the first furrow crosses, but is 
shallow in the middle, the second is less distinct, the third is 
fainter still; the basal lobes cannot be seen. When the head 
has grown to the length of 1 mm. the anterior furrow alone is 
distinct, and the glabella is much as in the adult. 
The pygidium in the larva of this size (1 mm. long ) is pro- 
portionately shorter and the rachis more prominent than in the 
adult, and the marginal fold and cusps are quite distinct; three 
transverse undulations divide the rachis into four somites ; 
these divisions except remnants of the anterior one disappear 
in the adult, as does also a minute spine at the back of the third 
lobe from the front. The embryonic pygidium is indicated by 
the low and weak part of the rachis behind the small spine 
visible on this tailpiece. 
An example 1} mm. long of the pygidium of this variety 
bears four cusps at the posterior border, but this extra armament 
is rare; it may be compared with var tricuspis found by 
Brogger in the Paradoxides bed of Krekling, in Norway,* 
which has a central cusp between the two usually found. 
Agnostus similis Hartt, which Walcott makes a synonym of 
A. Acadicus might belong here but for its want of spines on 
the caudal shield. 
War. Vin._pl. xv., iio. 6; 
Agnostus vir. Trans. Roy. Soc. Can., vol. iii., pt. iv., p. 69, 
pl. vili. fig. 3. 
2 Agnostus similis, Hartt, Acad. Geol., 2d ed., p. 606. 
This is a larger form than the type, and differs in the more 
conical glabella, with the spine set further forward, in the com- 
*On Paradoxidesskifrene ved Krekling, p. 87, Tab. vi., fig. la. 
