CRUSTACEA. 29 



The outline of this cephalon is nearly crescentic, its length one-half its width, 

 and the gen al angles produced into conspicuous spines. The glabella 

 with that of D. Hausmanni, both in proportions and character of lobation. li> 

 frontal lobe hears three circular depressions, one on the axial line and one near 

 each of the lateral extremities; of these the former is largest and deepest. 

 The eyes are large and elevated, the orbital sulcus unusually broad and con- 

 spicuous; the cheeks slope abruptly in front, and more evenlj toward the genal 

 angles, and their surface is marked by irregular, conspicuous wrinkles, except 

 where it is crossed by the broad, smooth sulcus in which the facial suture lies. 

 The border is a very characteristic feature, being moderately broad, smooth and 

 concave on the sides, narrowing near the extremities of the frontal glabellar 

 lobe and produced in front into a sub-nasute extension, .lust below the lateral 

 extremities of the glabella begins a series of marginal crenulations or alternate 

 thickenings and thinnings, which become more conspicuous anteriorly. 

 The border thus has a character similar to that of /) anchiops, and is 

 of especial interest as suggesting the inception of that peculiar frontal orna- 

 mentation which is more fully developed in Odontocephaltts. There is no doubt 

 that this is the cephalon of D. pleuroptyx, as in one well-defined example the 

 parts have been found in conjunction. The cephala which were referred to 

 this species on plate lxxiv of the third volume of the Palaeontology of New 

 York, with the possible exception of figures 5 ami !•. must therefore belong to 

 another species. No similar heads have been found in the Helderberg mount- 

 ains. The original pygidium of D. micrurus, Green, is known to have come 

 from Schoharie, although the author regarded it as from Trenton Falls. At 

 Schoharie the large pygidia which agree with the type of D. micrurus are not 

 uncommon, and the cephala with which they are there associated, and those in 

 the western extension of the Lower Helderberg formation, in Herkimer county, 

 should probably be regarded as belonging to this species. The cephala 

 described here as that of D. pleuroptyx appear to he of rare occurrence at 

 Schoharie, and that species seems to have been most abundant in the 

 Shaly Limestone, and D. micrurus in the Lower Pentamerus Limestone. 

 The entire young individual figured on plate lxxiv (Pal. N. V.. vol. iii). may 



