94 SYSTEMATIC ARRANGEMENT 



than in the other species of this group ; the three central lobes are nearly of an equal size, 

 and the eye reaches not only beyond the second, but even beyond the fourth, and almost as 

 far as the broad flat margin. The posterior angle is moderately pointed, probably as long 

 as four or five articulations. The large trilateral caudal shield has an a.xis of from nineteen 

 to twenty-two joints, and from fourteen to fifteen lateral ribs. It is, however, moderately 

 convex, and extended in a flat border at the circumference, which forms a carinated angle, 

 |)ointed at the extremity. The surface is finely granulated wherever the shell is preserved, 

 but smooth when it is absent ; the lateral ribs, in the latter case,do not appear to be 

 grooved, but merely flattened, but they have a double granulated I'idge. of unequal height, 

 when the shell is present. 



Remarks. 1. Brongniart, who knew this species b}' fragments only, represented the caudal shield 



as beino- rounded at the posterior part, which probably may be in consequence of the defective state of 

 his specimen. I therefore do not consider this caudal shield different from that represented by 

 Sternberg and Schlotheim, as Milne Edwards supposes (see p. 312 of his work). 



2. The angles of the cephalic shield are wanting in Count Sternberg's figure ; they are, however, 

 present in the perfect specimens which I saw at Berlin. A well-preserved head is very rare. 



3. Asaph, auriculatus, Dalm. [Paltead. 6. 6. 3), which is based upon Count Sternberg's unknown 

 individual (see his work, p. 80, Table II, Fig. 2), is beyond a doubt a smaller, younger specimen of 

 Ph. Haii.mumni, and therefore cannot be admitted as a species. 



11. Ph. caudatti.'s : Ocuhs miuorihus, nee anticum nee posticum limbum scuti cephalici attiugen- 

 tibus ; rhachi cauda; 14 annulata, costis lateralibus octo, limbo in apice acuminato. Long. 2-3". 



Uef.—Triloh. caudat. Brunn, Kjob. Scllsk. Skrift. N. S. i, 392. 3. Parkinson, 0/y. 

 Bern. PL XVII, Fig. 7. Schloth. Nachtr. 35. 11 (or 21. 4). 



Asaph, ccmd. Brongn. Crust, foss. 22. 4, PI. II, Fig. 4, a-c ; PI. Ill, Fig. 9. Dalm. 

 Palcead. 42. 2, and 65. 2, Tab. II, Fig. 4. Green, Mon. of TrU. 50. Buckl. 

 Bridg. Tr. PL XLV, Figs. 9-11, and PL XLVI, Figs. 11, 12. Murchis. Sil. Sj/sf. 

 ii, 654, PL VII, Fig. 8, a. 



Asajih. tuberoilato-caudatim, Murchis. Sil. Si/st. ii, 654, PL VII, Fig. 8, b. Milne Edw. 

 Crust, iii, 308. 2 (specimens with the granulation well preserved). 



l,oc. In a gray limestone in England (Dudley, Ludlow), in Sweden (Gothland), and in 



North America (Lockport). 



This species is nearly allied to the following one, but has characteristic distinctions ; 

 the anterior tubercle of the head is very large, and laterally very much produced forwards, 

 by which the eyes are thrown back; the latter are smaller than in P. Ilausmanni. 

 They do not reach beyond the anterior margin of the second lobe of the head, and only 

 just reach the front at the posterior part ; the posterior angles of the cephalic shield reach 

 to the centre of the body rings, the latter are gradually more pointed towards the posterior 

 part. The caudal shield has an axis with about fourteen joints, which is distinctly rounded 

 at the posterior part, and does not pass into the spine at the extremity ; there are eight ribs 

 on the sides, each divided by a diagonal furrow ; the broad expanded margin is lengthened 

 into a moderately long point at the posterior part, which is usually nearly as long as one 

 half the length of the axis. 



