NATURAL SCIENCES OF PHILADELPHIA. 265 



vex curve ; lateral margins contracted behind the anterior lateral angles, and 

 converging a little posteriorly, for about two-thirds the entire length, thence 

 more abruptly to the posterior extremity, which is transversely truncated, 

 and provided on each side with a minute, slightly projecting point ; while still 

 farther forward on each lateral margin, there appears to be traces of another 

 minute slightly projecting irregularity of outline. Around the anterior and 

 lateral margins, there is a more or less distinct sulcus, behind which the pos- 

 terior margin is flattened. Within this marginal sulcus there is, on each side 

 a little behind the middle, au oblique eye-like depression. 



Thorax wider than long, the length being to the breadth, as 21 to 28, near- 

 ly once and a half as long as the cephalic shield ; mesial lobe as wide anteriorly 

 as the posterior extremity of the glabella, and very slightly broader near the 

 middle, where it is about three-fourths as wide as the lateral lobes, from 

 which it is only separated by narrow, rather shallow furrows most convex 

 along the middle and flattened on each side ; segments not clearly seen in the 

 specimens examined. Lateral lobes somewhat more depressed than the mesial 

 one, and sloping very gradually to the lateral margins. Segments equalling 

 the antero-posterior diameter of the posterior lateral lobes of the glabella ; 

 each curving abruptly backwards at the outer extremity, and terminating in 

 a flat, sharply pointed, or lanceolate projection, most produced in the posterior 

 ones ; provided with a deep, well defined, longitudinal furrow, which starts 

 from the anterior side of the inner end, and passes at first a little obliquely 

 outwards, and then straight outward, slightly nearer the posterior than the 

 anterior margin, to the middle of the flattened scythe- shaped outer ends, where 

 they usually curve a little backwards and become obsolete. 



Pygidium nearly semielliptic, or subtrigonal, the anterior lateral angles be- 

 ing rounded, and the lateral margins converging to the more or less pointed 

 posterior extremity, with a broad convex curv.e ; slightly longer than the ce- 

 phalic shield, and rather more than two-thirds as wide ; mesial lobe somewhat 

 more convex than, and two-thirds as wide as the lateral lobes, gently rounded, 

 and tapering gradually to the posterior extremity, where it is apparently con- 

 tinued into an abruptly projecting caudal appendage ; segments 12 to 13, 

 straight well' defined (excepting near the termination) by distinct furrows, 

 which are deeper on each side than at the middle. Lateral lobes with eight 

 or nine well defined arched segments, which become more oblique posteriorly, 

 and are defined to near the edge of the smooth margin ; each divided by a 

 furrow deeper than those between, and like those of the thoracic ribs, the 

 anterior division being slightly shorter than the other. 



Surface (of cast) smooth, excepting traces of small, scattering tubercles on 

 the anterior lobe of 'he glabella. 



Entire length of the largest specimen seen (exclusive of the little caudal ap- 

 pendage, the length of which is unknown), 4*93 inches. Length of pygidium, 

 1-43 inches ; breadth of do. 2 inches ; breadth of its axillary lobe, 0*57 inch. 

 Length of thorax, 2-10 inches; breadth of do. 2-83 inches; breadth of its 

 mesial lobe, 080 inch. Length of cephalic shield, 1-40 inches; breadth of do. 

 3 inches ; length of posterior lateral spines, near - 95 inch ; length of glabella 

 exclusive of neck segment, 1-30 inches ; anterior breadth of same, 1-35 inches ; 

 posterior breadth of do. 0*84 inch. Length of eyes,0 '39 inch; distance of 

 same from posterior margin of cheeks, 42 inch. 



Named in honor of Prof. James D. Dana, of New Haven. 



We have described this fine species in as much detail as possible, because 

 it is somewhat nearly allied to several of the already described species. Per- 

 haps it is most nearly allied to the well known European D. caudata of Bru- 

 nich, with which it agrees in size, form aud many of its details. In the first 

 place, it differs, however, from that species in having the anterior margin of 

 its cephalic shield decidedly more rounded than even the variety or form re- 

 garded by Mr. Salter as the female, while it shows no marginal rim (as seen 

 from above) extending around the front of the glabella. Again, the eyes, in- 



1865.] 



