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, hearing 

 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 ami 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. 



