CRUSTACEA. 
51 
spines, giving the crescentic caudal extremity from which the specific name 
is derived. 
Axis relatively narrow, having less than one-fourth the width of the shield 
on the anterior margin; margins approximating with a slight inward curve 
to the apex which is broad, smooth, somewhat elevated, and distant from 
the posterior margin. Annulations from nine to eleven with indications of 
others on the terminal portion, visible only on casts of the under surface 
Pleura, composed of eight flattened annulations and traces of a ninth, which 
are abruptly bent backward near the margin. When the crust is well-pre¬ 
served the surface of each annulation bears a fine impressed line extending 
its whole length. 
Hypostoma elongate-sub-triangular or sub-linguiform. Anterior margin arcuate, 
antero-lateral angles auriculate. Centrum moderately convex, bearing 
posteriorly two pits connected by a faint groove, and separated from the 
posterior lingua by a conspicuous sulcus. The posterior margin appears to 
be smooth, but probably bears normally, one, three or five spinules. Lateral 
margins abruptly deflected dorsally. 
Surface Ornamentation. The surface of the glabella, cheeks, margin and mar¬ 
ginal denticulations is covered with faint obsolescent tuberculations. The 
entire test is very strongly punctate and the openings of the vertical tubules 
are visible everywhere upon the upper surface as well as upon casts of the 
under surface. 
Dimensions. Fragments of this species show a wide variation in size. 
Entire individuals are however of comparatively rare occurrence, and of 
these one of average size measures as follows: 
Body. Cephalon. Thorax. Pygidium. 
Length_ 80 mm. 24 mm. 36 mm. 20 mm. 
Width_46 mm. 46 mm. 44 mm. 37 mm. 
The largest entire individual observed has a length of 95 mm., and the 
smallest a length of 50 mm. 
